Error adding attachments in OWA

We have Exch 5.5 sp4 installed on NT 4.0 sp6a. We are running OWA on same server. All is working fine. I have one user who sometimes has problems adding attachments via OWA. Have check permissions – all ok. I can logon as that user from different computer and have had no problems. End user is using IBM T20 tp with W2k sp3 and IE 6.0. User does not user Outlook at all. Connects via a 56k dial up account. User sends and receives emails with no problems – just sometimes with attachments.