I had a support case 00868482 and after several back and forth configuration files and a couple webex the support tech found
DE312152 - Unable to Log into CEM After Upgrade
In the release notes of 10.5.2 sp2
I'm trying to get application of the sp2 into our v10.0 to v10.5.2 sp1 effort and it looks like we are going to need to run through our testing which will cost us several weeks worth of effort.
If I can get sp2 installed and if that fixes the access to the CEM interface, I'll post the results here.