Have you checked in (say) /etc/default/saslauthd that the mechanism you want to use (GSSAPI) is enabled?

At least for my Debian setup, I need to have MECHANISMS="PAM" there.

If you are sure that this is the case, stop your system-owned saslauthd and run it manually in the foreground in debug mode and hope that it shows when your client connects and why it fails:

saslauthd -d
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