The XMLSerializer needs permissions to the Windows\Temp directory to work as it compiles an assembly on the fly. Without permissions you may get file i/o exceptions raised during serialization.
Usually the error looks something like:
File or assembly name [tempfilename], or one of its dependencies, was not found.
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.
Exception Details: System.IO.FileNotFoundException: File or assembly name [tempfilename], or one of its dependencies, was not found.