Thank you for your response!
No, I've tried the flatpak version as well - as single user also. It's all the same. But it might be related to Linux Mint not automounting drives out of the box, yet I had it mounted and the link was right.
When I drag and drop a file from my home-folder, there is not a link like you would type it in, but kind of a symbolic link, e.g. it says :/1205c1802fad4bada9a132b3f9ad129c for a pdf file which is stored in /home/user/documents.
I've tried rights management changes, tons of options. It does not work from any drive with Linux Mint. Perhaps it's different with other distributions. this I don't know.
I've described it in detail here: NOT similar: Link to local file with Linux Mint does not work including log file and all.
Best regards,
Wolfram