Service Provider is throwing an error when attempting to consume the assertion generated by Siteminder. The assertion data looks correct in the FWSTrace.log, but the assertion data taken from the Fiddler trace (http trace) cannot be decoded. This encoded data from Fiddler does not match what we see in FWSTrace. The exact same configuration is working as expected on the same version of the software in a separate environment.
The partnership had somehow become corrupted despite not showing any errors.
Release: MSPSSO99000-12.8-Single Sign-On-for Business Users-MSP
Duplicating the problematic Partnership, deactivating the original and activating the duplicate resolved the problem.