XML Schema Metadata Issue Error

I have an xml document and schema, and when I open an xml document in visual studio, I get a bunch of warnings like:

A permission request of type 'System.Security.Permissions.FileIOPermission, mscorlib, Version = 4.0.0.0, Culture = neutral, PublicKeyToken = b77a5c561934e089' failed.

Does anyone know what might be causing these warnings? This makes it difficult to validate an XML document. Also, when I hover over the squidgly line below the value of the schemaLocation attribute, the tooltip gives the error "The scheme referenced by this position in your document contains errors"

Thanks in advance!