A log file would help determine what’s going on here possibly. Click the Browse Datastore button on the Instances screen. Then AMP_Logs will have the latest logs. Share one where you tried to take a backup.
OMG how embarrassing that I have never noticed it before. In the instance itself there is an area “Individual Backup Limit (MB)” and “Backup Space Limit (MB)” which is set to 10GB by default; however, the backup is ~11GB in size.
Correct this value upwards and everything will work as usual with AMP.