403.16 Client certificate error BUT no Client Certificate is requi

We have ONE user for a SSL-enabled web application who is encountering the
following error message:

403.16 - Forbidden: Client certificate is ill-formed or is not trusted by
the Web server.

User is using MSIE, version unknown.

Application is on a Windows 2003, service pack 1 server using IIS 6.0.
Wesite does not have a "certificate trust list" enabled. Our web application
does require SSL. Our web application does NOT require any client
certificates, and is in fact set to ignore client certificates.

Any suggestions on what to do to diagnose this issue?