Backup fails with ERROR snapshot failed to start Reason cannot create more than 100 manual snapshots

Backup fails with "ERROR: run_snapshots(.\agent.py:880) (database: "dbname") snapshot failed to start. Reason: cannot create more than 100 manual snapshots"

Backup fails with the following errors:

ERROR: run_snapshots(.\agent.py:880) (database: "dbname") snapshot failed to start. Reason: cannot create more than 100 manual snapshots
ERROR: backup_function_internal(.\agent.py:1204) policy: "policyname". All Snapshots in policy failed!

Customer must have exceeded the limit of 100 RDS snapshot per account per region: http://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/CHAP_Limits.html
Some of the 100 RDS snapshots may be present in other CPM policies or created outside CPM - please check the "Snapshots" view in the RDS Dashboard.
As a temporary workaround you can contact AWS support and ask to raise the limit.

You should either decrease the retention and run Cleanup, or ask AWS to increase this limit:

Please note: decreasing retention will only affect new backups from that point on, so you probably will need to delete some manually (or ask AWS to increase the limit) before you'll be able to take new snapshots.