Honestly I do not know what the message exactly means. But I would know how to try to work around this:
If you are the only user (syncing across multiple devices): Go to Settings, choose 'Open database location'. Make a copy of the file called theocbase.sqlite (this is your database, and you'll need a copy just in case something goes wrong).
Go to the cloud symbol in the upper right hand corner of TheocBase, hover over it and choose 'Delete Cloud Data' at the bottom. It will ask for confirmation. After cloud data has been deleted, it will immediately ask you if you want to sync. Choose to do that. (this will upload your local data to the cloud, effectively creating a new syncfile).
This way you'll find out soon enough if the previous syncfile was corrupted, or that something is wrong in the TheocBase process.
If you are not the only user, you may want to ask them (before doing any of this) to find their databases as well, and rename them (like theocbase_old.sqlite). After you confirmed everyone did this, you do the thing outlined above. What happens is: if they open TB, an empty database will be created locally. Then when they sync, that will be filled with the new data you put into the cloud.