Client auth not working if entire site is requiring it

If the entire site globally reqires SSL client authentication, Qt SSL fails to validate the server certificate:

Tested (fails) on:

https://idp.swisssign.net/webapp-sp
https://idp.swisssign.net/webapp-sp
http://cashback.suisseid.ch/Wineshop/DE
http://www.buch.ch

But it works for the same EV certificate e.g. on: https://secure.swisspostbox.com

SSL Error messages: 11, 12, then Network Error 6:
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SSL Error
URL:
https://idp.swisssign.net/webapp-sp
Error Code:
11
Error Details:
The issuer certificate of a locally looked up certificate could not be found
Certificate Issuer
Organization:
SwissSign AG
Common Name:
SwissSign EV Gold CA 2009 - G2
Location:

Organizational Unit:

Country:
CH
State or Provive:

Certificate Subject
Organization:
Die Schweizerische Post
Common Name:
idp.swisssign.net
Location:
Bern
Organizational Unit:

Country:
CH
State or Provive:
Bern

SSL Error
URL:
https://idp.swisssign.net/webapp-sp
Error Code:
12
Error Details:
No certificates could be verified
Certificate Issuer
Organization:
SwissSign AG
Common Name:
SwissSign EV Gold CA 2009 - G2
Location:

Organizational Unit:

Country:
CH
State or Provive:

Certificate Subject
Organization:
Die Schweizerische Post
Common Name:
idp.swisssign.net
Location:
Bern
Organizational Unit:

Country:
CH
State or Provive:
Bern

Network Error
URL:
https://idp.swisssign.net/webapp-sp
Error Code:
6
Error Details:
The SSL/TLS handshake failed and the encrypted channel could not be established. See SSL-Errors above.
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