So you're getting an error saving module settings in the contact form. All features use the same ModuleSettings.aspx page, does the error happen when saving settings for other features?
What version of mojoPortal and what db platform are you using?
It could be a bug in Mono, but if it is, its already fixed in Mono svn trunk, as I just tested it on my machine using pgsql and it worked.
Depending on your version of mojoPortal, it could be something already fixed there too. You might try getting mojoPortal from svn trunk using
svn checkout https://forgesvn1.novell.com/svn/mojoportal/trunk
Then build the mojoportal.mds solution in MonoDevelop. If you get any errors about unrecognized web.config sections, just comment them out in Web.config. I got these errors before but reported it and its now fixed in mono svn trunk.
You might want to look into building Mono from svn instead of using the release version.
Hope it helps,