same problem here. Deleting SerenityDB + restarting the web client service didn´t work.
I searched the web but the only promising hit ( http://blog.docx.org/2013/02/22/vmware-vsphere-meldet-fehlerhaftes-ssl-zertifikat/ ) didn´t work, too...
btw. which browser did you use? At first the web client worked for me using IE 8 32bit @ Win 7 Pro 64bit. The problem started right after I tried to connect using google chrome 31.0.1650.57m on the same machine, because I wanted to know if that works so that I can connect to the vCenter Server from my linux box, too.
I was using Chrome for my web client connection. I still haven't found a fix for this issue, and apparently we are the only ones on the planet that have come across this. I don't remember all my steps while getting used to the new stuff, but at some point I was using IE as well. None of the big 3 browsers work for me.
I suppose this isn't a current issue anymore, but I'm answering anyway in case anyone else experiences this issue.
I ran into this when testing an IPv6 setup. It occured once I configured an A and AAAA record for the ESXi hosts, which was necessary since the console plugin is broken (like a lot around the Web Client, but that's a different story...)
Bottom line: make sure there are no duplicate DNS entries for your ESXi hosts or the vCenter server, which means no second IP address for the same hostname. A hostname lookup must not return more than one IP address, no matter if A or AAAA.