In case others are having this problem with Outlook Windows Access, see <!mskb=924334>Microsoft Knowledge Base Article 924334<!/mskb> - a patch is required on the Server. Then S/MIME must be reinstalled on the Outlook Web Access client via the Options dialog. If Server admin doesn't wish to install the Server patch, and you are not using encryption or digital signing, you can remove S/MIME as explained in the 4th post in this MSDN thread

FWIW most of my crashes have been with OWA Premium, not OWA basic.