I met the same issue described in this VMware KB, unfortunately none of the solutions worked for me.
Symptoms
- When logging in with the vSphere Client you see a Security Warning dialog with a message similar to:An untrusted SSL certificate is installed on “<hostname>” and secure communication cannot be guaranteed. Depending on your security policy, this issue might not represent a security concern. You may need to install a trusted SSL certificate on your server to prevent this warning from appearing.
- You cannot click any of the dialog buttons or move the dialog window.
But I figured out a workaround that solved the problem for me – Add ‘-i‘ to ignore the certificate check. You can either run it through command line or add it into the shortcut.
Jackie Chen's IT Workshop 
nice one – been bugging me and this is the only solution that worked! Many thanks.
Nice…resolved for me 🙂
Tried doing as suggested but its not working for me. Any suggestions?
Try it in command line to see if it works.
worked for me as well. So I guess I need to install a new certificate then, right?
Most likely that you are using a private certificate, and the private CA certificate is not installed in the trust root on your machine.