LibreOffice 7.4.5 spelling issue with dict-de-de-frami-2017-01-12.oxt

In my working environment I cannot install software, so I set up LibreOffice inside my Documents folder.
Unfortunately the German dictionary seems missing, so I tried to install dict-de-de-frami-2017-01-12.oxt.
However that never worked: Installation begins, but aborts, and the result is some half-way installed German dictionary.
Interestingly I copied the portable folder from Documents to Downloads, and then I could install the extension, and I verified it's working.
So I copied the directory back to the Documents folder, but then the extension is missing again (Writer complains, and spelling errors remain undetected).
My Documents folder is synced to some server here; could that be the problem, or some security software blocking access?