As reported by Ramses Rodriguez Martinez in Debian BTS: mount.cifs manpage states the credential file format is: username=value password=value workgroup=value however, it should be: username=value password=value DOMAIN=value ....From what I see in mount.cifs.c, our user is right..:-)
You are perfectly right! Fixed with commit 3b7f8a759 in master and pushed to v3-4-test and v3-3-test. Closing out bug report. Thanks for reporting!