Hi,
That stack trace doesn't show any mojoPortal code only Mono code so I'm not sure what to tell you.
I know that when I compile Mono from source code to setup ASP.NET, I have to compile libgdiplus as it is needed for handling of image files.
To be honest I think Mono is still pretty rough around the edges which is why there are not many hosts offering it yet. They are on the verge of a new release of Mono that may be better but then it will depend on the host upgrading it. I cannot tell from this error what the real Mono version is at your host as Mono is just mimicing the ASP.NET version in the web page error.
If the host is actually claiming to support mojoportal then I would ask them for help assuming they have been able to get it working before.
Hope it helps,
Je