The case of an unresolvable error in DPM 2012 SP1 UR3
One of the issues closed in update rollup 2 for System Center Data Protection Manager 2012 SP1 was the following:
Issue 5: When you use Data Protection Manager 2012 Service Pack 1 for tape backup, you receive the following error message:
The operation failed because of a protection agent failure. (ID 998 Details: The parameter is incorrect (0x80070057))
So long so good. This error seems to be getting in and out and, basically, was introduced in DPM 2010, where you could not backup too many items to tape, then was fixes in a QFE, then was back again, until finally (or so we think) was fixed in the above mentioned update rollup.
This is why I found it very strange when I got confronted with this error again, on a system running DPM 2012 SP1 UR3, to be more specific - the second edition of the UR3.
One of the data sources got stuck and did not want to be written to tape even after a couple of retries. The only solution that finally worked was to dismiss the alert and create a manual recovery point of type Tape, which used a different (a later) disk-recovery point created while I was struggling with the alert.
So, if you run into a similar problem, try to manually backup your data source to tape from a different recovery point than the one giving you an error.