This looks like some sort of memory or freelist corruption in BackupPC_tarExtract:

 *** glibc detected *** /usr/bin/perl: realloc(): invalid next size: 0x093bd438 ***

This error is deep inside perl, and normally shouldn't happen when you are running any sort of perl code. But... a dynamically linked module like BackupPC::XS could be a culprit.  I did run tests on the underlying library using the address sanitizer checks in LLVM, and it was clean.  But maybe I didn't have enough test cases.

The challenge is to replicate this on my machine (or get me access to a machine that shows the problem).  I'm guessing it is subtly dependent on the exact tar file being piped into BackupPC_tarExtract from the client.

The good new is the error appears to happen very early in the backup, which makes debugging a bit easier.  Can you set the XferLOG level to say 9 and send me the whole XferLOG file?  (Don't copy it to the list.)