Since we upgrade our DNN community edition from 6.0.1 to 7.1.0 we have following error:

We have done everything serializing every class there is in our app_code. We changed everything from viewstate to session, then it works. We would like to know what happened or what was changed.

this is the error stack trace:

DotNetNuke.Services.Exceptions.PageLoadException: Error serializing value 'E2E.Modules.Reservations.RES_ComplexInfo' of type 'E2E.Modules.Reservations.RES_ComplexInfo.' ---> System.ArgumentException: Error serializing value 'E2E.Modules.Reservations.RES_ComplexInfo' of type 'E2E.Modules.Reservations.RES_ComplexInfo.' ---> System.Runtime.Serialization.SerializationException: Type 'DotNetNuke.Security.Roles.RoleController' in Assembly 'DotNetNuke, Version=7.1.0.2676, Culture=neutral, PublicKeyToken=null' is not marked as serializable. at System.Runtime.Serialization.FormatterServices.InternalGetSerializableMembers(RuntimeType type) at System.Runtime.Serialization.FormatterServices.GetSerializableMembers(Type type, StreamingContext context) at System.Runtime.Serialization.Formatters.Binary.WriteObjectInfo.InitMemberInfo() at System.Runtime.Serialization.Formatters.Binary.WriteObjectInfo.InitSerialize(Object obj, ISurrogateSelector surrogateSelector, StreamingContext context, SerObjectInfoInit serObjectInfoInit, IFormatterConverter converter, ObjectWriter objectWriter, SerializationBinder binder) at System.Runtime.Serialization.Formatters.Binary.ObjectWriter.Write(WriteObjectInfo objectInfo, NameInfo memberNameInfo, NameInfo typeNameInfo) at System.Runtime.Serialization.Formatters.Binary.ObjectWriter.Serialize(Object graph, Header[] inHeaders, __BinaryWriter serWriter, Boolean fCheck) at System.Runtime.Serialization.Formatters.Binary.BinaryFormatter.Serialize(Stream serializationStream, Object graph, Header[] headers, Boolean fCheck) at System.Runtime.Serialization.Formatters.Binary.BinaryFormatter.Serialize(Stream serializationStream, Object graph) at System.Web.UI.ObjectStateFormatter.SerializeValue(SerializerBinaryWriter writer, Object value) --- End of inner exception stack trace --- at System.Web.UI.ObjectStateFormatter.SerializeValue(SerializerBinaryWriter writer, Object value) at System.Web.UI.ObjectStateFormatter.Serialize(Stream outputStream, Object stateGraph) at System.Web.UI.ObjectStateFormatter.Serialize(Object stateGraph) at System.Web.UI.ObjectStateFormatter.System.Web.UI.IStateFormatter.Serialize(Object state) at System.Web.UI.Util.SerializeWithAssert(IStateFormatter formatter, Object stateGraph) at System.Web.UI.HiddenFieldPageStatePersister.Save() at System.Web.UI.Page.SavePageStateToPersistenceMedium(Object state) at System.Web.UI.Page.SaveAllState() at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) --- End of inner exception stack trace ---