When a VCCS (Veeam Cloud Connect Service) for any reason becomes unreachable to the appliance for a period of time, the appliance will no longer be able to perform the copy jobs and throw the following error: 'Cloud repository is unavailable due to service provider restrictions'. Even after the VCCS has become reachable again, the appliance might still throw this error.

Solution:

1: Open the Veeam Backup and Replication Console
2: Select the option "Backup Infrastructure"
3: Select "Service Providers"
4: You should see your service provider listed on the right. Please right click it and select 'Properties'

5: Click 'Next' through the properties wizard and DO NOT make any changes.
6. When you have finished going through the properties wizard, go back to your backup copy job and select 'Sync now'

After this procedure the backup copy jobs should run again as intended.