I just upgraded from Gpg4Win 2.3.4 to version 3.0 and now I cannot sign or check signatures anymore. I use Win 7 (64bit).
Since Kleopatra did not start (see https://dev.gnupg.org/T3475) I started it manually. Then I wanted it to check a signature and used the menu File->Decrypt/Verify Files...
As result I only get this cryptic error message before Kleopatra crashes:
In Gpg4Win 2.3.4 this problem does not occur.