Rebooting in Safe Mode made the Dashboard visible again, so it must be Plugin releated.
Did´nt had time to investigate further, just noticed the stable 6.12.1 relase and upgraded to that.
The Dashboard was visible again with all Plugins, but have some displaying issues like mentioned in this thread:
Don´t have many installed, my best guess is my tasmota power plugin. I´ll try that first. Is there a list of incompatible Plugins with 6.12.?