Hello @caustic and @Florian
We have prepared test build of the connector app for Nextcloud v.29: Artifact · ONLYOFFICE/onlyoffice-nextcloud@e4f9234 · GitHub
Could you please install it and check the situation? You need to disable and remove current connector app and after that install the test build manually: ONLYOFFICE Api Documentation - Nextcloud ONLYOFFICE integration app
After that clear browser cache and check the situation.