"the user you are attempting to 'become' (user name here) does not exist on this system."

We have used Pegasus for many years and are having an issue we cannot seem to troubleshoot. We are currently having to look at mail through our backend (hosting) and cannot get into any user accounts. HELP!

Maybe this thread can be helpful for you: https://community.pmail.com/index.php?u=/topic/11535/pmail-migration-requires-a-pegasus-mail-username-to-access-my-mailbox

This problem does not appear out of the blue. Something has changed. What happened prior to this problem? A known cause is a change in the path of the Pegasus Mail installation or the location of the mailboxes. Did anyone do any file or directory renaming or relocating?

Is your Pegasus Mail installation a multi-user installation...
...on a single PC?
...an installation with mailboxes on a different drive or network device?
...a shared installation of Pegasus Mail on a home or business network?

Just to be clear. Become is used for Identities within a users mail directory.
Example: My primary user has the default and gmail Identities, and I can use the become option to switch between them...

In a multi-user setup, you need to start an instance of PMAIL using the ID.
Example. Have a test setup.
In PMAIL/MAIL directory
cat PMAIL.USR
A;Admin;Mail Administrator
U;Jack;Jack Hill
U;Jill;Jill Hill

can select the different users
C:\PMAIL\Programs\WINPM-32.EXE -I ADMIN
or
C:\PMAIL\Programs\WINPM-32.EXE -I JACK
or
C:\PMAIL\Programs\WINPM-32.EXE -I JILL

The PMAIL.INI file in each users mail directory has identity info.

Might check rights for the MAIL subdirectorys, since a user would need to have read/write access to the other users mail directories if wanting to open them.