I just experienced the same issue when upgrading from View 5.0 on vSphere 5.0 to Horizon View 5.2 on vSphere 5.1 U1. I was able to accept the certificates for various components except vCenter. The security event log on the vCenter server showed a failed login attempt using the configured credentials in Horizon View. When I attempted to modify the vCenter settings to change the account credentials in Horizon View Administrator console, it required me to accept the certificate before it would modify the settings. At this point I had to contact VMware support.
VMware support used ADSI Edit to modify the ADAM database. I believe the problem was probably only that the account did not have my domain prefix in the username field, but they ended up modifying quite a few more settings (account name, encrypted password, ssl cert settings). Long story short: they changed the UNIQUEID of the vCenter server, added a new vCenter server and then transferred the settings back to the original entry before deleting the new entry and restoring the UNIQUEID in the ADAM database.