No, the svn_authz is for the AuthzSourcePolicy.
I think that simply there were still the old svn changesets in the cache, and they were associated with the repos id which is now a mercurial one. We should probably try to reproduce such a scenrio (changing type from svn to hg) and check if the advice given is correct (and see if a resync works correctly in such situation, i.e. that it removes the old svn changesets).
Sorry to still not have the DB backend for Mercurial (#8417), hence no possibility to search for now. Incidentally I'm working in this area today, but no promise as the when this might become available in a branch near you ;-)
Also, the problem you had with the repr was already mentioned in #10174 and I think that we should indeed use some kind of safe_repr in the error reporting code.
