Basically the destination does not match the database records. They should always match.
Duplicati makes use of a local database for each backup job that contains information about what is stored at the backend.
Don’t use old database versions from backups. If database is lost, recreate from destination.
Never store multiple backups in the same remote location. Backup data files from multiple backup jobs will interfere with each other and will result in failed backup operations. Always create a new remote folder for each backup job that you set up.
Regarding the small number of files, it might be normal (as updates to a backup can be small, however you might want to make sure Remote volume size on Options screen is near 50 MB default, because one common error is to set this way too large inadvertently, causing troubles.
If none of the above apply, we will probably need a Complete log from failure and run before. Sometimes failures will not leave the usual log, but it can be significant if that’s known. There’s About → Show log → Stored that may show prior errors from a failure leading up to mismatch.
If it is a rare problem without known interference as described above, it may need extra logging.
log-file=<path> with log-file-log-level=Retry would be a good start, but heavier logs are possible.