This is probably because the SSL-thumbprint in the vCenter DB was not updated accordly. We had a similar issue when we replaced certificates a while ago.
Update the DB or use the HostReconnect.pl script as instructed by this KB article:
http://kb.vmware.com/kb/2006210
I'm aware the article only mentions 5.0 but I think the issue is still present in 5.1, at least on U0.