When a SQL Backup Server Instance has too much activity history it can cause issues with loading and showing the history on the SQL Backup GUI. This can showcase itself as the Server Instance having a green spinning circle icon that always loads.
The easiest way to fix this issue is to manually delete said activity history. This involves running a deletion script against the SQL Server Instance with the SQL Backup Server Components.
You can read more about how to do so here: https://documentation.red-gate.com/sbu9/troubleshooting/deleting-backup-and-restore-history-manually
To prevent this issue from re-occurring, it's possible to have the SQL Backup Pro graphical user interface (GUI) delete the backup and restore history from the msdb database and SQL Server Compact database for each SQL Server (Tools > Server Options > File Management) on a set schedule. For more information, see File management options.