I have now seen this occur on multiple places trying to upgrade or install from scratch Portal 10.5.1. As soon as a our certificate is applied to Portal it is completely broken. You can no longer log in and it requires going back. The issue is described in the Common problems site: Common problems and solutions—Portal for ArcGIS (10.5.x) | ArcGIS Enterprise. However, there is no solution offered.
How can this be fixed? The certificate all worked in version 10.4.1, 10.5 but do not work in 10.5.1. Do some of the newly added parameters need to be adjusted? It would be nice that instead of just indicating an issue is known a solution is actually offered, just rolling back is not a solution.