This issue came up while integrating HCatalog with our environment. HCatalog jars are added to the pig command-line with -Dpig.additional.jars but fails (exception below). When added to the pig classpath the error goes away.
We identified the issue as deserialization using the root class loader, not the context class loader set when the thread is created. This causes HCatSchema which is serialized into the context to fail deserialization in the thread.