Cisco VPN Client hangs at "Opening cert store"

I finally found out what was causing this. A single answer by Chad Roeder on the Cisco support forums had the right solution:

1. Open Microsoft Management Console (type mmc in Run or search).
2. Add Snap-in "Certificates" (File -> Add/Remove Snap-in... -> [find and add "Certificates] -> Choose "My User account" -> OK)
3. Go to the folder Personal/Certificates
4. Remove any unwanted certificates (for me it was certificates added by Fiddler to enable SSL traffic debugging)
5. Consider repeating step 2-4 but using "Computer Account" instead of "My User account".

I hope this helps making the solution more visible in online searches. :)

To mitigate this issue within Fiddler, either use the Fiddler CertMaker Extension (which does not add the certificates to your store in the first place) or use the QuickExec box below the Fiddler Web Sessions list to run the following command:

prefs set fiddler.CertMaker.CleanupServerCertsOnExit True