"The custom form could not be opened. Outlook will use an.."

Greetings!

First time poster to this forum.

I'm looking for some assistance with the following error:
"The custom form could not be opened. Outlook will use an Outlook form instead. The object could not be found.".

What has been done so far?
- Using Outlook Client 2002 (SP3).
- There is a Teradata server (Windows server)that sends e-mail notifications (alerts) out to 4 users using a distrubtion list. ALL 4 users receive this exact error mentioned above. All of sudden, this error has started to appear when they first click on the e-mail in their INBOX. It was working fine before until now.
- I had asked the server admin to add my name to the group of 4 (distrubtion list). I too, receive the same error when I receive an e-mail notification (alert).

Is this an Outlook client issue? If so, what can be done (steps) so this error no longer appears?

Appreciate your time and response(s). Thank you!