OUTLOOK97/2003 WITH EXCHANGE 5.5

We have a user with Outlook 97 connecting to an Exchange 5.5 mailbox on the server.
User has been testing Outlook 2003 using her account still connecting to the 5.5 mailbox.
When she returns to her original PC and uses OL’97 again it crashes and can’t access the mailbox. This occurs on various 97 client machines and not just hers.
I’ve tried using 2003 to access the account and then 97 just after that which occasionally works for a while but then it crashes again.
Is there any known problems with swapping between Outlook 97 and 2003 clients into the same Exchange account?