What if you were to create a user "test" with a password "testpass", as in the
Postfix docs.
Code:
perl -MMIME::Base64 -e \
> 'print encode_base64("\0test\0testpass")'
AHRlc3QAdGVzdHBhc3M=
Try that and see if it works. Some Googling about referenced unfortunate character combos in password or user id being interpreted as octal numbers rather than ascii characters, which'll give you a hash which doesn't make sense. Start from known good.