I have posted 2 other messages in regards to this issue,
but I think I was able to narrow down the problem now.

(sorry for long message, but I wanted to answer all
questions you may have).

This is the issue:

I started with a blank Post item. I added an Action
called "Collaborate", which is basically a Reply
with "Attach link to original message" option.

I published this form to a folder.

Then I changed the icons of this form(both small and
large) to something I created and published this form to
the same folder with a different name.

Finally I created two items based on each of these forms
and posted them to that folder.

If I open the item created with the first form and
sellect "Collaborate" from the Actions menu, it works ok.

If I open the item created with the second from (the form
with different Icon) and try that action I get "The object
is not valid." error.

The problem is clearly because of the icons.

Both forms perform ok on Outlook2002. The problem happens
only in Outlook 2003

Have you ever experienced this?

Thanks,
Artem