Hmm. Unfortunately, that's no guarantee there isn't a HW failure. The diagnostics code has improved with later and later versions of EXOS.
As Drew mentioned, there is one occurrence of this error in our internal bug tracker on a 12.5.x version, and this does smell like a HW failure...but it might be worth it to move to EXOS v15.6.4.2-patch1-7 to see if the error recurs/newer diagnostics code can pick up a failure.