I think i found the problem,
I did a mistake: the dictionaries installed on the kobo clara hd and forma were not identical.
Somehow there is a dictionary named "eng-deu.index" (Foldername freedict-eng-deu-1.8.1-stardict), I assume it is not optimized for koreader. I don't remember where I got it from. Its size is around 130MB.
https://freedict.org/de/downloads/ does not even have any file that large. None of the devices could use this dict.
I have moved some files before a while between an encrypted cloud folder and my local drive, somehow some stuff got corrupted I assume (weirdly eReader related too as it was calibre library with calibre having problems with it). Maybe the dict got corrupted in the process too.
after disabling it the dictionary started to work as usual.
Thank you for your reply