Operation could destabilize the runtime

On version 7.3, we are now getting "System.Security.VerificationException: Operation could destabilize the runtime" when running what seem like simple tests. The errors only occur when running on the build server though, when run on the client the tests run fine.

Example stack trace:
System.Runtime.Serialization.DataContractAttribute..ctor()
System.RuntimeTypeHandle.CreateCaInstance(RuntimeType type, IRuntimeMethodInfo ctor)
System.Reflection.CustomAttribute.GetCustomAttributes(RuntimeModule decoratedModule, Int32 decoratedMetadataToken, Int32 pcaCount, RuntimeType attributeFilterType, Boolean mustBeInheritable, IList derivedAttributes, Boolean isDecoratedTargetSecurityTransparent)
System.Reflection.CustomAttribute.GetCustomAttributes(RuntimeModule decoratedModule, Int32 decoratedMetadataToken, Int32 pcaCount, RuntimeType attributeFilterType, Boolean isDecoratedTargetSecurityTransparent)
System.Reflection.CustomAttribute.GetCustomAttributes(RuntimeType type, RuntimeType caType, Boolean inherit)
System.RuntimeType.GetCustomAttributes(Boolean inherit)
System.Xml.Serialization.XmlAttributes..ctor(ICustomAttributeProvider provider)
System.Xml.Serialization.XmlReflectionImporter.GetAttributes(Type type, Boolean canBeSimpleType)
System.Xml.Serialization.XmlReflectionImporter.ImportElement(TypeModel model, XmlRootAttribute root, String defaultNamespace, RecursionLimiter limiter)
System.Xml.Serialization.XmlReflectionImporter.ImportTypeMapping(Type type, XmlRootAttribute root, String defaultNamespace)
System.Xml.Serialization.XmlSerializer..ctor(Type type, String defaultNamespace)
System.Xml.Serialization.XmlSerializer..ctor(Type type)
HoraceMann.Auto.Services.Classing.Tests.Unit.Data.XMLDataLoaders.LoadClientInformationListFromXML(String filename) in c:BuildsHM.Marketing.CustomerCareCenterCCC - Main - Gated BuildSourcesSourceHoraceMann.Auto.Services.Classing.Tests.UnitDataXMLDataLoaders.cs: line 20