How recently have you done the onlyOffice NextCloud sync? We had issues with the software being outdated and no longer supported on a setup we did a few months ago.
Discussion
I am running a few instances now. They are all at the latest versions.
I run them on NixOS. Nextcloud is run manually not with the Nix module and onlyoffice is run via the docker image.
Sovran_SystemsOS has it already build it. However, to run onlyoffice I have custom .nix file not in the repo.
https://git.sovransystems.com/Sovran_Systems/Sovran_SystemsOS