The offline file (offline.db) need to be deleted manually.
Click on File - Data Sources and edit your current data source. At the bottom left, you will retrieve the Data source ID. You would need this Data source ID to delete the appropriate offline.db file.
Data Source ID
Now, close Remote Desktop Manager for Mac.
You can find the offline.db in Finder by accessing the Library folder. To do so, you need to click on the Go menu, press and hold the Option key on your keyboard and select Library.
You can then follow the path Application Support/com.devolutions.remotedesktopmanager/GUID-DatasourceID and delete the offline.db file.
Restart Remote Desktop Manager for Mac. This will force the application to recreate the offline file.