Welp, looks like I'm upgrading my backup server from BackupPC 3 to 4 this weekend. 4.x has been out of beta for 5 years, it ought to be reliable by now.
Upgraded one of our Debian boxes to 11 and hit a bug in the BPC rsync Perl module on Deb10 only triggered by the newer rsync on the Deb11 client. Really surprised nobody has resolved this when the bug was discovered two years ago, before Deb11 was released.
So the solution is to upgrade my backup server to Deb11 and BPC 4.
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