I can't change the trust level of imported keys, "Allgemeiner Fehler" this applies to all keys, I can change the trust level with gpg itself via console.
Thanks for your report. There was a Bug in GpgME++ that caused this when GnuPG 2.1 was used. This bug has been fixed in GpgME 1.8 and so it's fixed in Kleopatra 3.0