activating encryption leads to a boot-loop

[Seegras]

Device: Xperia X
Android version: Explosive_Lobster_F5121-06022021-R.suzu

Describe the bug:
When activating encryption, the phone boots, seems to counts backwards from 100% down, and then up again, during something like 6 hours (empty userdata: on other phones this takes only a few minutes even with user data), and stops.

After that, it boot-loops after the sony screen.

It can be saved by going into flash-mode and flash userdata.img again (which obviously reverts any encrypting it might have done).

I tried this now twice, with the exact same results.