You offline cache may be out of sync with the content of the data source. Press CTRL + F5 to refresh the local cache.
The offline file (offline.db) need to be deleted manually.
Click on File - My Data Source Information and now hover over the file size. This will give you the full file path, something like %LocalAppData%\Devolutions\RemoteDesktopManager\GUID-DatasourceID\offline.db.
My Data Source Information
Close Remote Desktop Manager, delete the offline.db file and restart Remote Desktop Manager. This will force the application to recreate the offline file.