I sat down again and implemented a hopefully proper fix this time.
Successfully tested on OS X and Windows with Kerberos turned on.
Please see if this fixes both issues for real:
"Installer": http://code.x2go.org/releases/binary-macosx/x2goclient/previews/4.0.4.1/x2go...
Signature: http://code.x2go.org/releases/binary-macosx/x2goclient/previews/4.0.4.1/x2go...
Checksums: http://code.x2go.org/releases/binary-macosx/x2goclient/previews/4.0.4.1/x2go... http://code.x2go.org/releases/binary-macosx/x2goclient/previews/4.0.4.1/x2go... http://code.x2go.org/releases/binary-macosx/x2goclient/previews/4.0.4.1/x2go...
Mihai