[quote=“Bas_Koning, post:2346, topic:30020”]
Reinstalled unreal 4.23.1, added advanced sessions to project plugin folder, add class, compile, no more error. I think my perhaps my older version of unreal was still polluted with the old version of advanced sessions (I simply removed it from the engine plugin folder). I had several crashes with that older unreal as well: perhaps setting some folder/files it was working on to ‘read only’, which would create that very error message. So that could also be it. (but then: I did try it out on freshly created projects as well… ) Anyway, its working now 
Deleting intermediate folders generally fixes issues like that, the generated meta code can get stale.