Unsubscribe not working: Users not found in database

[a-goodwin]

Our users are unable to unsubscribe.
This is the page that comes up for them when they try:

There were errors removing you from our mailing list(s):

User not found in database. You should not receive any more email.

WHat should I do from here?

Thanks
Ariane

[a-goodwin]

Would appreciate any help or suggestions on what we can do for the unsubscribers on our list.

Thanks,
Ariane