I may have figured out the FSID (File System ID) errors with the new version of MEGAsync. It wasn't an issue with version 4.12.2. Starting on v5, it happened daily. Sometimes MEGAsync has an issue and sometimes it doesn't. I tested with an empty new folder. If it has an issue right away, I have to remove the syncs and re-add them, and it scans and adds the new files. If everything seems fine, the FSID errors show up and the syncs get disabled the next morning, and I have to remove and re-add them.
I use a cache drive and the files go to that first. The cache drive has the filesystem btrfs and and my regular drives have xfs. I disabled the cache drives for my syncs that go to MEGA a couple days ago, and I haven't seen the FSID error yet.