Problems to save Content in the Panel

I have a problem in a kirby2 installation with two languages on some pages.
the default language is german, the second is english.

if I want to save content on the english-version of a page via the panel – and the panel jumps back to the previous position after saving.
If I save content on the German version, the save is accepted and the content is there.
on other sites, saving works in english and German, fine.

what could be the problem?
I have already deleted the txt file of the page.en.txt and had it created again.
the rights of the txt are also identical to the others.

Are there any extra .txt files, e.g. without language code or leftover file meta data files?