Hi, thanks @jvstratum for following up. I already contacted support, but normally they respond withing a day, now they are not responding at all.
So little bit worried here. Upgrading to Joomla 5 breaks the installation as the Acymailing plugins still (!) rely on outdated classnames (JPlugin).
The new "Behaviour - Backward Compatibility" will take care of this once the installation is finished, but as this plugin is not available on the upgrade of Joomla it crashes the site and leaves it in a crashed state.
So before updating Joomla first disable all acymailingplugin, do the upgrade (Joomla will automatically enable the "Behaviour - Backward Compatibility" plugin, and publishthe acymailing plugins again.
Then testing can begin.
I really do not understand why Acymailing is still using these old pre Joomla 3.10 class names. These should have been taken care of a long time ago.
Anyway.
on we go, hope this helps somebody who is also trying to test Joomla 5