[Bug] Registration fails, raising a non-descript error toast.

[RokeJulianLockhart]

What The Problem Is

When I attempt to register for an account, I am disallowed:

Screencast_20260521_194605.webm

How To Reproduce It

#!/usr/bin/env xdg-open
window.location.assign('https://mermaid.ai/app/sign-up?redirect=%2Fapp%2Fuser%2Fbilling%2Fcheckout%3Futm_campaign%3Doss_coupon%26utm_medium%3Dbanner_ad%26utm_source%3Dmermaid_live_editor%26coupon%3DarDfyFT8%26tier%3Dplus#email:~:text=Email,Re%2Denter%20password');
document.querySelector('.\[\&\[type\=\'email\'\]\]\:border-neutral-200').value += 'nx09y8rt@rokejulianlockhart.addy.io';
document.querySelectir('.text-base').click();
document.querySelectorAll(
	'div.gap-1:nth-child(2) > div:nth-child(2) > input:nth-child(1), ' +
	'div.flex-col:nth-child(3) > div:nth-child(2) > input:nth-child(1)'
).forEach(input =>
	input.value = 'KitHndK$UeBgW#EFXik2PnSsr88tILbXGh^HXXDsZAzm2l#yCnrGqK1AhshfnLQMmqiDfroAI2M@aFAP4jiHLk&00x0pCrRi^XJrB2C%JFymE6Zkj5be4MdpjfG0PjMt'
);

The Expected Behaviour

I should be able to register.

My Environment

1. Details

   #!/usr/bin/env sh
   rpm -qf "$(command -v firefox-nightly)" \
   	--queryformat "$(
   		cat <<'EOF'
   Name: %{NAME}
   Version: %{VERSION}
   Release: %{RELEASE}
   Architecture: %{ARCH}
   Install Date: %{INSTALLTIME:date}
   Size: %{SIZE}
   Signature: %{SIGPGP:pgpsig}
   Source RPM: %{SOURCERPM}
   Build Date: %{BUILDTIME:date}
   Build Host: %{BUILDHOST}
   Packager: %{PACKAGER}
   Vendor: %{VENDOR}
   EOF
   	)"$'\n' | yq -P

2. Name: firefox-nightly
   Version: 153.0a1
   Release: 20260520124049
   Architecture: x86_64
   Install Date: Thu 21 May 2026 02:01:27 BST
   Size: 387616918
   Signature: (none)
   Source RPM: firefox-nightly-153.0a1-20260520124049.src.rpm
   Build Date: Wed 20 May 2026 15:55:51 BST
   Build Host: 1a0a267703b6
   Packager: (none)
   Vendor: Mozilla

Additional Information

• The relevant HAR (from //*[@id="toolbox-tab-netmonitor"]) is available at gist.github.com/RokeJulianLockhart/064926adb35cbc80bd15521841368176, because it is too large to paste.

• The relevant //*[@id="toolbox-tab-webconsole"] output, after a clear(), is:

  19:46:41.156 XHRPOST
  https://mermaid.ai/rest-api/users/signup
  [HTTP/2 400  240ms]
  
  19:46:41.402 Feature Policy: Skipping unsupported feature name “cross-origin-isolated”. api.js:1:40579
  19:46:41.402 Feature Policy: Skipping unsupported feature name “autoplay”. api.js:1:40579
  19:46:41.402 Feature Policy: Skipping unsupported feature name “keyboard-map”. api.js:1:40579
  19:46:41.402 Feature Policy: Skipping unsupported feature name “xr-spatial-tracking”. api.js:1:40579
  19:46:41.402 Feature Policy: Skipping unsupported feature name “cross-origin-isolated”. api.js:1:40579
  19:46:41.402 Feature Policy: Skipping unsupported feature name “autoplay”. api.js:1:40579
  19:46:41.402 Feature Policy: Skipping unsupported feature name “keyboard-map”. api.js:1:40579
  19:46:41.402 Feature Policy: Skipping unsupported feature name “xr-spatial-tracking”. api.js:1:40579
  19:46:41.403 Feature Policy: Skipping unsupported feature name “cross-origin-isolated”. api.js:1:40768
  19:46:41.403 Feature Policy: Skipping unsupported feature name “autoplay”. api.js:1:40768
  19:46:41.403 Feature Policy: Skipping unsupported feature name “keyboard-map”. api.js:1:40768
  19:46:41.403 Feature Policy: Skipping unsupported feature name “xr-spatial-tracking”. api.js:1:40768
  19:46:41.404
  Object { response: {…} }
  app.7vxQzMBi.js:5:238
  19:46:41.405 Expected declaration but found ‘undefined’.  Skipped to next declaration. sign-up:1:10
  19:46:41.407 GET
  https://challenges.cloudflare.com/cdn-cgi/challenge-platform/h/b/turnstile/f/ov2/av0/rch/8bg86/0x4AAAAAAANyydvnRfYewKyi/light/fbE/api/flexible?lang=auto
  [HTTP/3 200  51ms]
  
  19:46:41.500 GET
  https://challenges.cloudflare.com/cdn-cgi/challenge-platform/h/b/cmg/1
  [HTTP/3 200  19ms]
  
  19:46:41.693 XHRPOST
  https://challenges.cloudflare.com/cdn-cgi/challenge-platform/h/b/flow/ov1/892471307:1779386900:kdz4pwvWZG-r73LvWeajh2bslM2zS5wYUfAuMneRe3Y/9ff5beccd9e7e8fe/kYDL8hgaqn7gaeTgErUBShVIuiGWqQDbTs8dLOe5NyE-1779389201-1.2.1.1-bMxumocPBukwUgNwY5z9tLxxuMYmnDAcnInSJZ5H43FDqguaFnY9r_q2end0aact
  [HTTP/3 200  213ms]
  
  19:46:41.941 Window.fullScreen attribute is deprecated and will be removed in the future. flexible:1:30478
  19:46:41.941 InstallTrigger is deprecated and will be removed in the future. flexible:1:30478
  19:46:41.941 onmozfullscreenchange is deprecated. flexible:1:30478
  19:46:41.941 onmozfullscreenerror is deprecated. flexible:1:30478
  19:46:41.979 <empty string> flexible:1:200751
  19:46:42.046 WEBGL_debug_renderer_info is deprecated in Firefox and will be removed. Please use RENDERER. flexible:1:200751
  19:46:42.286 WebGL context was lost. 2 flexible:1:200751
  19:46:42.298 Error in parsing value for ‘font-size’.  Declaration dropped. flexible:1:6
  19:46:42.298 Error in parsing value for ‘position’.  Declaration dropped. flexible:1:9
  19:46:42.298 Error in parsing value for ‘font-size’.  Declaration dropped. flexible:1:6
  19:46:42.298 Error in parsing value for ‘position’.  Declaration dropped. flexible:1:9
  19:46:42.298 Error in parsing value for ‘font-size’.  Declaration dropped. flexible:1:6
  19:46:42.298 Error in parsing value for ‘position’.  Declaration dropped. flexible:1:9
  19:46:42.298 Error in parsing value for ‘font-size’.  Declaration dropped. flexible:1:6
  19:46:42.298 Error in parsing value for ‘position’.  Declaration dropped. flexible:1:9
  19:46:42.298 Error in parsing value for ‘font-size’.  Declaration dropped. flexible:1:6
  19:46:42.298 Error in parsing value for ‘position’.  Declaration dropped. flexible:1:9
  19:46:42.298 Error in parsing value for ‘font-size’.  Declaration dropped. flexible:1:6
  19:46:42.298 Error in parsing value for ‘position’.  Declaration dropped. flexible:1:9
  19:46:42.298 Error in parsing value for ‘font-size’.  Declaration dropped. flexible:1:6
  19:46:42.298 Error in parsing value for ‘position’.  Declaration dropped. flexible:1:9
  19:46:42.298 Error in parsing value for ‘font-size’.  Declaration dropped. flexible:1:6
  19:46:42.298 Error in parsing value for ‘position’.  Declaration dropped. flexible:1:9
  19:46:42.298 Error in parsing value for ‘font-size’.  Declaration dropped. flexible:1:6
  19:46:42.298 Error in parsing value for ‘position’.  Declaration dropped. flexible:1:9
  19:46:42.298 Error in parsing value for ‘font-size’.  Declaration dropped. flexible:1:6
  19:46:42.298 Error in parsing value for ‘position’.  Declaration dropped. flexible:1:9
  19:46:42.299 Error in parsing value for ‘font-size’.  Declaration dropped. flexible:1:6
  19:46:42.299 Error in parsing value for ‘position’.  Declaration dropped. flexible:1:9
  19:46:42.299 Error in parsing value for ‘font-size’.  Declaration dropped. flexible:1:6
  19:46:42.299 Error in parsing value for ‘position’.  Declaration dropped. flexible:1:9
  19:46:42.299 Error in parsing value for ‘font-size’.  Declaration dropped. flexible:1:6
  19:46:42.299 Error in parsing value for ‘position’.  Declaration dropped. flexible:1:9
  19:46:42.299 Error in parsing value for ‘font-size’.  Declaration dropped. flexible:1:6
  19:46:42.299 Error in parsing value for ‘position’.  Declaration dropped. flexible:1:9
  19:46:42.299 Error in parsing value for ‘font-size’.  Declaration dropped. flexible:1:6
  19:46:42.299 Error in parsing value for ‘position’.  Declaration dropped. flexible:1:9
  19:46:42.299 Error in parsing value for ‘font-size’.  Declaration dropped. flexible:1:6
  19:46:42.299 Error in parsing value for ‘position’.  Declaration dropped. flexible:1:9
  19:46:42.299 Error in parsing value for ‘font-size’.  Declaration dropped. flexible:1:6
  19:46:42.299 Error in parsing value for ‘position’.  Declaration dropped. flexible:1:9
  19:46:42.299 Error in parsing value for ‘font-size’.  Declaration dropped. flexible:1:6
  19:46:42.299 Error in parsing value for ‘position’.  Declaration dropped. flexible:1:9
  19:46:42.299 Error in parsing value for ‘font-size’.  Declaration dropped. flexible:1:6
  19:46:42.299 Error in parsing value for ‘position’.  Declaration dropped. flexible:1:9
  19:46:42.311 Request for the Private Access Token challenge. flexible:1:200751
  19:46:42.354 XHRGET
  https://challenges.cloudflare.com/cdn-cgi/challenge-platform/h/b/pat/9ff5beccd9e7e8fe/1779389201711/fae9695f589740c2cf5272ce2ecab39fa048f525449307b8042619b0a2ab365b/FuQepw3MalEfcpX
  [HTTP/3 401  20ms]
  
  19:46:42.548 This page is in Quirks Mode. Page layout may be impacted. For Standards Mode use “<!DOCTYPE html>”. flexible:1:200751
  19:46:42.585 GET
  https://challenges.cloudflare.com/cdn-cgi/challenge-platform/h/b/d/9ff5beccd9e7e8fe/1779389201712/hKt_suZ6CT4g4a2
  [HTTP/3 200  20ms]
  
  19:46:42.704 XHRPOST
  https://challenges.cloudflare.com/cdn-cgi/challenge-platform/h/b/flow/ov1/892471307:1779386900:kdz4pwvWZG-r73LvWeajh2bslM2zS5wYUfAuMneRe3Y/9ff5beccd9e7e8fe/kYDL8hgaqn7gaeTgErUBShVIuiGWqQDbTs8dLOe5NyE-1779389201-1.2.1.1-bMxumocPBukwUgNwY5z9tLxxuMYmnDAcnInSJZ5H43FDqguaFnY9r_q2end0aact
  [HTTP/3 200  128ms]