I have upgraded to gnucash 3.4 in Windows10. After changing the locale to LANG=de_DE LANGUAGE={LANG} and reopening the file, all transaction dates have been erased. Downgrading to 3.3 and changing the locale solved the problem for now.
I think this is a duplicate of bug 797109. Can you try a recent nightly build from https://code.gnucash.org/builds/win32/maint?
(In reply to John Ralls from comment #1) > I think this is a duplicate of bug 797109. Can you try a recent nightly > build from https://code.gnucash.org/builds/win32/maint? I have upgraded to 3.5 and it works. Sorry for the slow response, I was travelling and did not find the time to check this.