Error importing Internet Certificates into Notes

I receive the following error when importing an internet certificate:

Cannot add certificate from import file

The internet certificates are working in IE, then exported as DER.Binary, DER.64 and PKCS 7.

But I receive the above error on each import try.

The Internet certificates are delivered by a Danish phone company (TDC), which are not derived from a commonly known root certificate like Verisign et.al. But they are succesfully imported into Notes using the native Notes Web browser.

What could be the problem?