I was finally able to fix it with a tip from this answer. I added the following <add>
line in the web.config
:
<configuration>
<system.web>
<compilation debug="true" targetFramework="4.0">
<assemblies>
<!-- etc... -->
<add assembly="System.Data.Linq, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" />
<!-- etc... -->
</assemblies>
</compilation>
<system.web>
</configuration>