I could solve the issue today for my Onlyoffice flatpack version, on Linux Mint 22.2 Cinnamon (Nemo as file manager), by changing the template-file, like this:
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activate root privileges / „admin rights“ for Nemo: open terminal and type in the command: pkexec nemo ➞ there should open a Nemo window with the red bar with something like „root privileges“
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in that window find the template files for Onlyoffice. In my system and as I have the flatpack they are located in:
/var/lib/flatpak/app/org.onlyoffice.desktopeditors/x86_64/stable/141a88e75e9ea2a1bfaf3724affa5c055005882e1e7f8f7ee4c64271b712b93f/files/bin/opt/onlyoffice/desktopeditors/converter/empty/ -
there choose the folder of the language your Onlyoffice is set up (in my case it is: de-De for German, etc.), where you find the template files, example: new.docx – copy that file somewhere, open it and adjust your prefered settings, save it, and copy-overwrite it back into the former template folder. That should do it.
To deactivate the root privileges for Nemo, just close that certain window.
(Background information: I am totally new in Linux and no kind of expert. So more experienced people, please correct me in case I created some unaware problems or risks in the background. The issue itself works for me now.)