I figured it was a mono thing because it never happens locally when testing under IIS.
I'm always afraid to upgrade mono to fix one problem because it always causes two new ones! Our portal has been up for almost a year now, and users use it to approve invoices digitally, etc, so it's now a critical system to the company, so any mono upgrade is a tense moment...
BTW the conversion from Cuyahoga to mojoPortal went so smoothly. I switched over Monday morning and haven't had a single call. The next part is training the users to post their own files/articles instead of calling IT.
Thanks Joe!