The site's identity certificate may have been revoked. You should contact the operators of the site for information.

Your other computer probably has OCSP disabled. OCSP lets Certificate Authorities alert you if a previously-issued certificate is revoked. Enable it by opening Options (Firefox button>Options, or, if you use the menu bar, Tools>Options, Tools>Preferences, or Edit>Preferences, whichever works on your system). Open the Advanced pane, then the Encryption tab (see the first picture). Click "Validation." Set the options as shown in the second picture, but you may adjust the "Response Signer" option. Ensure, however, that the "Service URL" box is not empty. If it is, try using another response signer. When you are done, click OK, then close the Options window.

Note that the button styles and icons may look different in your setup.

Thanks for your reply. I did try this, but it has no result. What does help, is when I uncheck all boxes (see picture). But I don't think that is save.

I am getting errors in both Firefox and Internet Explorer so I think this is a problem on their site that they need to fix. For information how to get help, see
http://www.funda.nl/contact/

@knorretje. Did mention it at the helpdesk, just a while ago. Wondering what the answer will be. As soon as I hear something I will mention it here to.

This site www.funda.nl now has a new certificate from Thawte.

I just logged in, everything is working again. Did not get a reply, but who can tell how many e-mails were send to the webmaster