SessionState problem after upgrading

Description

Partner reports they get the exception below - after submitting a form (EPiServer.Forms.Implementation.Elements.FormContainerBlock) on a page and the page is reloaded when the site uses SQL Server SessionState:
<sessionState mode="SQLServer" sqlConnectionString="..." cookieless="false" timeout="720" />

See comments for more information

[SerializationException: Type 'Episerver.Marketing.Connector.Framework.Data.EntityProfile' in Assembly 'Episerver.Marketing.Connector, Version=5.3.0.0, Culture=neutral, PublicKeyToken=null' is not marked as serializable.]
System.Runtime.Serialization.FormatterServices.InternalGetSerializableMembers(RuntimeType type) +965
System.Collections.Concurrent.ConcurrentDictionary`2.GetOrAdd(TKey key, Func`2 valueFactory) +83
System.Runtime.Serialization.FormatterServices.GetSerializableMembers(Type type, StreamingContext context) +238
System.Runtime.Serialization.Formatters.Binary.WriteObjectInfo.InitMemberInfo() +194
System.Runtime.Serialization.Formatters.Binary.WriteObjectInfo.InitSerialize(Object obj, ISurrogateSelector surrogateSelector, StreamingContext context, SerObjectInfoInit serObjectInfoInit, IFormatterConverter converter, ObjectWriter objectWriter, SerializationBinder binder) +711
System.Runtime.Serialization.Formatters.Binary.WriteObjectInfo.Serialize(Object obj, ISurrogateSelector surrogateSelector, StreamingContext context, SerObjectInfoInit serObjectInfoInit, IFormatterConverter converter, ObjectWriter objectWriter, SerializationBinder binder) +91
System.Runtime.Serialization.Formatters.Binary.ObjectWriter.Serialize(Object graph, Header[] inHeaders, __BinaryWriter serWriter, Boolean fCheck) +751
System.Runtime.Serialization.Formatters.Binary.BinaryFormatter.Serialize(Stream serializationStream, Object graph, Header[] headers, Boolean fCheck) +217
System.Web.Util.AltSerialization.WriteValueToStream(Object value, BinaryWriter writer) +2117

[HttpException (0x80004005): Unable to serialize the session state. In 'StateServer' and 'SQLServer' mode, ASP.NET will serialize the session state objects, and as a result non-serializable objects or MarshalByRef objects are not permitted. The same restriction applies if similar serialization is done by the custom session state store in 'Custom' mode.]
System.Web.Util.AltSerialization.WriteValueToStream(Object value, BinaryWriter writer) +2274
System.Web.SessionState.SessionStateItemCollection.WriteValueToStreamWithAssert(Object value, BinaryWriter writer) +50
System.Web.SessionState.SessionStateItemCollection.Serialize(BinaryWriter writer) +758
System.Web.SessionState.SessionStateUtility.Serialize(SessionStateStoreData item, Stream stream) +291
System.Web.SessionState.SessionStateUtility.SerializeStoreData(SessionStateStoreData item, Int32 initialStreamSize, Byte[]& buf, Int32& length, Boolean compressionEnabled) +80
System.Web.SessionState.SqlSessionStateStore.SetAndReleaseItemExclusive(HttpContext context, String id, SessionStateStoreData item, Object lockId, Boolean newItem) +731
System.Web.SessionState.SessionStateModule.OnReleaseState(Object source, EventArgs eventArgs) +832
System.Web.SyncEventExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +194
System.Web.HttpApplication.ExecuteStepImpl(IExecutionStep step) +213
System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +91