Memcache issue when upgrading to rc6

[Bofus]

If memcache is enabled in Configuration->Horde->Caching (tab):

cache-driver = memcache = "Use a distributed Hashtable Server" Then select "Hashtable" as the memorycache.

The system will throw an error in the web interface:

Horde\HashTable\Driver\Memcache::get(): Argument #1 ($key) must be of type string, array given, called in /var/www/horde/vendor/horde/cache/lib/Horde/Cache/Storage/Hashtable.php on line 69
in /var/www/horde/vendor/horde/hashtable/src/Driver/Memcache.php:51

1. require_once() /var/www/horde/web/horde/admin/config/index.php:3
2. Horde_Themes_Image::tag() /var/www/horde/vendor/horde/horde/admin/config/index.php:157
3. SimpleXMLElement->addAttribute() /var/www/horde/vendor/horde/core/lib/Horde/Themes/Image.php:106
4. Horde_Themes_Element->__toString()
5. Horde_Themes_Image->__get() /var/www/horde/vendor/horde/core/lib/Horde/Themes/Element.php:109
6. Horde_Themes_Element->__get() /var/www/horde/vendor/horde/core/lib/Horde/Themes/Image.php:45
7. Horde_Prefs->getValue() /var/www/horde/vendor/horde/core/lib/Horde/Themes/Element.php:125
8. Horde_Prefs->_getScope() /var/www/horde/vendor/horde/prefs/lib/Horde/Prefs.php:306
9. Horde_Prefs->_loadScope() /var/www/horde/vendor/horde/prefs/lib/Horde/Prefs.php:399
10. Horde_Prefs_Cache_HordeCache->get() /var/www/horde/vendor/horde/prefs/lib/Horde/Prefs.php:454
11. Horde_Cache->get() /var/www/horde/vendor/horde/prefs/lib/Horde/Prefs/Cache/HordeCache.php:53
12. Horde_Core_Cache_Session->get() /var/www/horde/vendor/horde/cache/lib/Horde/Cache.php:113
13. Horde_Cache->get() /var/www/horde/vendor/horde/core/lib/Horde/Core/Cache/Session.php:101
14. Horde_Cache_Storage_Hashtable->get() /var/www/horde/vendor/horde/cache/lib/Horde/Cache.php:113
15. Horde\HashTable\Driver\Memcache->get() /var/www/horde/vendor/horde/cache/lib/Horde/Cache/Storage/Hashtable.php:69

Reverting to horde/horde rc5 and horde/core rc7 fixes this issue. One can also turn off the caching server options under the Caching tab.