For a while I have been experiencing an error when trying to use OMA. The error returned by IE was "A system error has occured" but not due to the usual cause in KB 817379.
The event log showed the following:
Event ID: 1503
An unknown error occurred while processing the current request:
Message: Index was out of range. Must be non-negative and less than the size of the collection.
Searching for event 1503 with MSExchangeOMA as the source revealed nothing relevant, but the message "Index was out of range..." lead me (in a roundabout way) to KB 319886 "E-mail message is not delivered from Exchange 2000 Server to Exchange Server 5.5 recipients". On the face of it the article appears completely unrelated but it states that individual mailboxes can experience problems if the homeMTA attribute for the user in AD is incorrect.
Following the steps in the article I found that the homeMTA attribute for my user account was indeed incorrect - presumably as a result of the problems I had experienced before Christmas.
Changing this to the correct value "CN=Microsoft MTA,CN=COLINW-SVR2,CN=Servers,CN=First Administrative Group,CN=Administrative Groups,CN=randomelements,CN=Microsoft Exchange,CN=Services,CN=Configuration,DC=randomelements,DC=me,DC=uk" (by copying it from a working account) and, hey presto, OMA functions for my user account again.