Up to this afternoon I was able to sign my documents with an advanced signature by Signtrust (Acrobat 11.0.10, Win 7 32 bit). Suddenly, without made any changes, I get the message "Die für das Unterschreiben ausgewählte Referenz ist ungültig" (engl. "the choosen reference for signing is not valid"). Signing with other tools or Acrobat 9 is still possible.
Signtrust is a 3rd=party security plugin to Acrobat. I do not know how it works. If it uses certificate-based authentication then that certificate or some other certificate used in the process may have expired.
Same problem here. All certificates (root, anchors etc.) are proven valid, though.
(Mac OS X Yosemite 10.10.2, Acrobat 11.0.10, SuisseID Qualified, purpose: non-repudiation)
Thanks for your help!
Meanwhile I solved the problem by deinstalling Acrobat XI and installing it again. Nevertheless, the cause of the problem remains unknown