2.9 - Unable to Enter Edit mode on appended instance

While working with 2.9 for a day now, i get the Message “Error: Unable to change modes - Entering Edit Mode failed”. The part i was about to edit was just appended and made local. I also wasn’t able to add more than 1 Material slot. I can delete and add them, but never more then one. Also the dropdown to change the selected material to another is deactivated oO