See if the troubleshooting steps in
Since this is only for a few specific clients, this one jumps out:
The user's browser does not have a preferred language set. To set a
preferred language in Internet Explorer, on the Tools menu, select Internet
Options. On the General tab, click Languages, and then click Add.
> Help me!
> I have a E2k sp2 oan a win2000 server. Everyone can access owa except the
> users that have windows 98. They can log in but after that it only says "
> loading" in the window where the mails shoult showup. They have used IE
> and 5.5.