I have this problem as well.
Found the Bug ID and tried the provided workaround but that solves not the issue.
I access the sponsor portal DNS name with http (e.g. http://sponsor.portal)
but get a redirect at first to https://sponsor.portal and this represents the Admin Portal certificate and I get a certificate error message (CN wrong). After confirm this wrong certificate it redirects to the final portal page.
If I click "Portal test URL" it does not work eather...
...and this is a an officail way to test the sponsor portal url - should work for sure and not by workaround (which mentioned also not work)
@cisco provided workaround does not work - plesae fix asap.