That sounds like the problem I was having. If so, try this:
If you are using the default keyring in UCSM, the UCSM CLI commands are:
scope security
scope keyring default
set regenerate yes
commit-buffer
Then try to reconnect.

I encountered the same issue.  I discovered we had created our own signed certificate by our CA and changed UCSM to use that certificate.  Once we had done that we deleted the default certificate since it was not needed.  Once I recreated the default certificate (it just had to exist) I was able to join UCS Central