Sounds like:
- Sync folder lost from settings · Issue #11411 · owncloud/client · GitHub
- Client no longer able to sync files with the server (Arch Linux) · Issue #11440 · owncloud/client · GitHub
- linux client (appimage) sync in pause · Issue #11455 · owncloud/client · GitHub
Linked issues document a workaround how to fix it and today’s 5.2.1 might include a fix that prevents it from happening again: