Thank you, you have saved me from pulling my hair out.

Updated web.config to add the line

<add assembly="System.Data.Linq, Version=4.0.0.0, Culture=neutral, publicKeyToken=b77a5c561934e089" />

to the system.web -> compilation -> assemblies section.

For some reason a 3.5 project adds this line but a 4.0 project does not.