Per the comments at the bottom of the 10.5 "Support and Compatibility" page the external access to pcm.ca.com problem is being worked on, and I have also highlighted it again:
I think the multiple login issue is only an intermittent internal problem for our SSO internal accounts
If after logging into the Communities you use the Compatibility Guide.link and you have authorisation to access the page it does not request login again, so I think Joshua hit that because his login does not have access.
So I think Hal is correct and CSO Support need to check Joshua's account permissions for the CG page.