Outlook 2003 - Unable to open digitally signed messages

When one of my users attempts to open a digitally signed message she gets an error that states: "Can't open this item. A required action was not successful due to an unspecified error."

The mails are not encrypted just digitally signed, Firewall is disabled by default for all users. I have tried all possible troublshooting suggested in various sites but I couldn't find a concrete solution to this problem.

I've tried to re-create Outlook profile, pst file, nothing worked.

I have un-installed, and re-installed office. I have run the repair tool,
and all the updates are installed.

The message is good, and the certificate is valid.  I had the sender send me
and the original recipient another message. I was able to open the message
with no problems, but the original recipient was not able to open it, and
received the same error again.

She cannot view the e-mail in the reading pane. When she tries to view it,
she gets the following message: "This item cannot be viewed in the reading
pane. Please open the item to view the contents."  whereas I am able to view it in my
reading pane.

Please assist
Who is Participating?
FreshandeasyAuthor Commented:

I have already looked into that question, here the scenario is different and the error message is also different, it doesn't help.
FreshandeasyAuthor Commented:
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