I have installed SP3 on several servers and workstations here on my network and I have indeed found a large problem.
If you are running the Exchange 5.5 admin console on a machine you install SP3 to, you might not be able to communicate with the exchange server again. In my scenario
We have 2 Exchange 5.5 servers, I have the admin console loaded on my workstation....after installing service pack 3 on my workstation I can no longer reach the Exchange server "B" but I can still reach Exchange server "A". Big problem is that the admin console (working of course) is requirred to backup using Backup Exec's Exchange agent. Since I installed SP3 on my backup server in my "B" office, I can no longer backup mailboxes on Exchange server "B". I would recieve "Communications Failure" anytime I clicked on "Bricks Level" under Backup Exec 8.6 (on either server I could connect to bricks level on Exchange "A" but not "B", also just opening Exchange admin on either server "A" or "B" I can view/admin "A" but not "B"

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I originally found out about this through a user forum at Microsoft.com but once I uninstalled the SP2SR1 from my machine (had to uninstall the newly upgraded IE version first) I was working fine, but thinking that the newer SP3 would be ok to install proved false. Whatever file from SP2SR1 that screws up Exchange Admin, is also screwed up in SP3. This was the problem/fix I found at ms.com and it proved correct for ALL 3 of the machines I was having the problem on.
I searched and searched for anything I could about a fix or even the error OFFICIALLY on ms.com but found nothing whatsoever, only from a user in a forum.
The problem also developes (as far as I have tested and read in forums) from installing SP2SR1