Hi KOMBiII,
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.