I think I found the problem: There was a Java pop-up that said there was a problem with the certificate and asked me to "always trust websites with this certificate". I checked it. Then, I was able to log in without the SSL error.
Could you confirm that checking it is the correct action, vs. fixing a certificate problem? ...Since the dashboard and SnapSwitch both login VIA SSL without problem, I'm concluding there is no problem with the certificate.