Client

Sometimes the client can get stuck in very, very long operations (especially if you move or changed large folders around). This is normal - you can check the progress of what's going on in /home/elvanor/.config/Nextcloud/logs/. If it still writes to a log there, everything is fine, you just need to wait.
There is also a nextcloud_sync.log file, located at /home/elvanor/.local/share/Nextcloud/. There is probably a bug there as this file should be present in /home/elvanor/.config/Nextcloud/.

Server

The Nextcloud server log (located at data/nextcloud.log) can grow very big and should be deleted from time to time.
Each file version (and deleted files) are kept in data/elvanor/files_versions and data/elvanor/files_trashbin. Those folders can grow a lot and should be purged as well, however they can be very useful in case of serious issues. For instance, at some point I had a corrupted Calibre database - Calibre could not load it and would crash everytime. Restoring a previous version fixed the issue.
To retrieve a previous version of a file, use the web interface, it's easy and more robust than trying to find the correct file directly in the files_versions directory (if at all possible).