I'm running into this too.  It works perfectly for me in an Incognito/InPrivate window though.  It also works when I delete my browser cache (Chrome/Firefox/Edge).  But then after a day or so....back to the same SAML error again unless I go to Incogn...

You can disable SAML/SSO just for RTMT (under System/Enterprise Parameters).  It's a crappy workaround, but it works for the time being until you can update CUCM.