Finally got some time again...
# rpm -Va
S.5..
/usr/lib/mono/gac/Accessibility/4.0.0.0__b03f5f7f11d50a3a/Accessibility.dll
S.5..
/usr/lib/mono/gac/Accessibility/4.0.0.0__b03f5f7f11d50a3a/Accessibility.dll.mdb
S.5..
Well, the one thing that's clear is "Invalid certificate received from server."
You said it works on other systems, so most likely there isn't an *actual*
problem with the certificate, so the question is why mono on the pi is
rejecting it.
Can you print the URL? From another system, we can