Weird thing is, the issue wasn't around prior, and 'suddenly' it was
there. Now I know we did not change anything on the serverside of
things, and the fact that it works for another IMAP enabled user
supports this theory. (I should note... the only other IMAP user
(while using Outlook) is running an Apple Mac, and thus the software
on the clientside is somewhat different)

The issue itself therefore seems to lie with Outlook itself for some
reason. The workstation has been patched with Microsofts 'latest and
greatest' on XP SP2 aswell as Office 2002, so it can't really be a
software issue. If it was, I'd expected it to pop up almost directly
after installing these patches. Since Microsoft releases patches on
the 2nd tuesday of the month (the last actual updates released were
provided around the 14th), and the problem was reported per the 22nd,
I doubt the issue is within these patches. Looking at the last
reported list of updates issued by Microsoft I note the following
updates being distributed:

KB936357: Update for Windows XP
KB923810: Securityupdate for Windows XP
KB943460: Securityupdate for Windows XP
KB939653: Incremental securityupdate for Internet Explorer 6 Service
Pack 1
KB941202: Securityupdate for Outlook Express 6 Service Pack 1
KB943552: Update Junkfilter Outlook 2003
KB943559: Update JunkfilterOutlook 2007
Essential update for Office 2003 (943452)

No actual Office 2002 fixes were released, and we're having issues
with Outlook, and not Outlook Express. If Outlook Express is the
culprit, it can only be KB939653 that causes the issue. Prior to that
the last fixes released were dated the 24th of october with this list:

KB939653: Incremental securityupdate for Internet Explorer 7 voor
Windows XP
Microsoft .NET Framework 1.1 Service Pack 1
KB928416: Microsoft .NET Framework 3.0: x86
KB917013: Windows Desktop Search 3.01 for Windows XP
KB928416: Languagepack for Microsoft .NET Framework 3.0: x64
Windows Server 2003 Service Pack 2 (32-bit x86)

All in all, again nothing that would warrent strange behaviour from
Outlook 2002. I have verified that appointments were planned after the
15th of november and prior to the 22nd of november and everything
worked normally. Looking at the timeframe of the release of the
updates (14th released, 15/16th working, 22nd not working), it is
feasable the updates for november may have had a hand in things,
although it seems like a long shot.

The user is in daily, and updates are automatically installed by WSUS
on a daily basis. Updates are approved or discarded as soon as a
notice e-mail arrives, which means they were processed on the morning
of the 14th of November. This in essence would mean that updates
installed that afternoon, or at shutdown, or at the very least the
15th. This would hint at the fact that it did work with the updates
installed.

Since when moving the mail around it does work all of a sudden, I
doubt there is actual form/data corruption active on the machine. But
for some reason it seems like the first time an incoming message is
processed it doesn't detect that it is a meeting request, and thus
doesn't display the buttons. Since I'm unaware what Outlook does
specifically to determine the nature of an incoming message, I'm a bit
at a loss as to where to specifically look for a solution on this.

Thnaks sofar for your assistance tho. It's greatly appreciated.