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I found the error.
I have an older Version of Django-Fiber installed. Which uses (and also older Version) of django-mptt to store stuff in the database. And this older (0.5.1) Version of mptt has a bug with their polish translations, that was fixed in this commit:
https://github.com/django-mptt/django-mptt/commit/4b6a9758396450651bc2d02b2c7d49bac6cd3f25
I updated mptt to version 0.5.5 and everything works fine now!
๐คAnton
Source:stackexchange.com