<p>My client gets a <code>sec_error_unknown_issuer</code> error message when visiting <a href="https://mediant.ipmail.nl" rel="noreferrer">https://mediant.ipmail.nl</a> with Firefox.
I can't reproduce the error myself. I installed FF on a Vista and a XP machine and had no problems. FF on Ubuntu also works fine.</p>

<p>Does anyone get the same error and does anyone have some clues for me so I can tell my ISP to change some settings?
The certificate is a so called wild-card SSL certificate that works for all subdomains (*.ipmail.nl). Was I wrong to pick the cheapest one?</p>

Which version of Firefox on which platform is your client using?
The are people having the same problem as documented
here in the Support Forum for Firefox
. I hope you can find a solution there. Good luck!
Update:
Let your client check the settings in Firefox: On "Advanced" - "Encryption" there is a button "View Certificates".
Look for "Comodo CA Limited"
in the list. I saw that Comodo is the issuer of the certificate of that domain name/server. On two of my machines (FF 3.0.3 on Vista and Mac) the entry is in the list (by default/Mozilla).