Comment 19 for bug 1923053

the issue in Comment 18 comes from mixing higher versioned clients, they are not downgradeable.

"That is because the sync database of 3.2.0-rc3 and 3.1.3 are incompatible. In general it is not supported to downgrade. To recover from that you need to delete the sync database while the client is not running. The sync database are the files beginning with ._sync* in your sync folder. The client will generate a new sync database on restart." from https://github.com/nextcloud/desktop/issues/2790#issuecomment-811872914