Trouble with forum inside another Web application
When you have a site with its web.config file and installed the forum under the site root, sometimes you will run into problems running the forum like:
System.Configuration.ConfigurationErrorsException: The value for the 'compilerVersion' attribute in the provider options must be 'v4.0' or later if you are compiling for version 4.0 or later of the .NET Framework.
The reason for these weird errors is mostly due to
web.config inheritance - the forum inherits the settings from your own web.config file which causes conflicts with its own settings.
Also, you may experience issue like
registration failures and
log in failures that present as "verification failure" or with WinXP the complete inability to enter a unique user name or password.
The solution: Break the inheritance
In your site (parent) web.config, wrap the following:
<location path="."inheritInChildApplications="false">
</location>
around
system.web, system.webServer, and system.codedom.
This should prevent inheritance from happening between the site and the forum.