Hmm,
I've never tried mono on Windows myself but to me this looks like a mono error. I'm assuming you are using the 1.x branch or release of mojoPortal and not the 2.x or 2.0 release, is that correct? If you are using the 2.x branch or release, it does not currently work on mono.
To be honest most of my testing has been using mono compiled on Suse from the mono svn repository rather than release versions of mono. In the past though I have often found problems with mono releases. They always encourage you to use the latest but often you see breakage from one release to the next unfortuantely.
I'm in the process now of setting up some extra virtual machines to test mono releases so I can better inform mojoPortal users which ones are good and which ones are not, but I don't have it all setup yet.
I just googled the error and found it was reported recently and is fixed in svn but probably the bug was in the release you are using:
http://www.mail-archive.com/mono-list@lists.ximian.com/msg18369.htmlThe error appears to have been reported by someone using the mono 2.0 stack/xsp2 so either the same error happens with xsp or maybe you are trying to use mojoPortal 2.x with mono which is not currently supported due to errors such as this.
Cheers,
Joe