So... if you want to do something right do it yourself.
I have a problem similar to the one described in this blog http://blog.isaacblum.com/2012/08/01/kaspersky-anti-virus-vmware-esxvmware-vsphere-client-osx/
But quite different - program called Device Lock cause this error.
By default then you install vi client first time you must install VMware deafult ceritificate.
And in my case this certificate issued not VMware but Device Lock O_O - (actually this programm remind me powerful virus -big damage,little use)
I disable Device Lock,Kaspersky and voila -install succefull!
I know it's an old thread but I had the same issue today while I was trying to deploy an ova template, getting both error messages "Unable to establish an SSL connection with vCenter Server", and then "Unable to validate that the OVF can be imported on the vCenter Server".
After trying everything online without any luck, I figured out (after like 3 hour) the issue was that my management PC (which I used to login to vCenter 5.5) is not joined to the domain. To solve this issue, I only pointed 'on the mgmt PC" to the DNS on my domain (I didn't join the mgmt PC to domain) and it was all good after that.
I also noticed that because my mgmt PC was not joined to the domain nor pointing to the domain DNS I was unable to open any of my VM consoles (I get an error "ssl verification failure for "ip" due to a host thumbprint mismatch") again after just pointing my management PC DNS to the domain, this issue disappeared.
Just wanted to share my experience with you guys, good luck!
Old thread but I ran into this & with the help of support discovered the solution. I had iPrism (Saint Bernard) software installed. This software is for content filtering that I installed to test. I forgot to remove it but when I did, vSphere client began working normally again. Since the tech support guy had this issue/resolution in his database, I figured I should add it here for others to find.
I had the same error. I have an iPrism Web Filter appliance from Edgewave The iPrism has a software option for laptops that is used to enforce web filter rules while outside the office. That software caused the "thumbprint mismatch" error as well as other network issues. Once I uninstalled St Bernard software, all was back to normal.