Comment out this line in your web.config file like so:

<!--<trust level="High" />-->

Additionally, if you installed blogengine.net in the root of your server and have problems with other asp.net applications, you can limit child applications from inheriting your root web.config by:

1. Put this right above the <system.web> line (~line 39):
   <location path="." inheritInChildApplications="false">
2. Put this right after the </system.web>: </location>

I hope this helps someone.