The screenshots show that the problem is not in the end-entity certificate (the actual server's certificate) but in the intermediate CA which issued it (the one just above): that CA is also shown with the "red cross". The error messages you see just tell you that the server's certificate cannot be used because its issuing CA is not trusted. To know why that issuing CA is not trusted, you have to click on it.
From my own systems, I see no problem with that issuing CA, and my browsers are happy with it (Firefox, Chromium on Linux; IE 8 on Windows XP). A possible reason may be a failure to obtain the CRL and/or OCSP information from the "DPWN Root", making it impossible for Safari to ascertain whether the "DPWN SSL CA" certificate was revoked or not (use of CRL and/or OCSP depends on the browser, the OS, and the configuration of both, so this may explain conflicting results).