Getting "Token Text in state Start would result in an invalid XML document. Make sure that the ConformanceLevel setting is set to ConformanceLevel"

Question

We started getting below errors while testing the Translation Manager on Tridion 2013 SP1, so when we send one component for translation it generate exception.

Technical details

System.ServiceModel.FaultException`1[Tridion.ContentManager.CoreService.Client.CoreServiceFault]: Token Text in state Start would result in an invalid XML document. Make sure that the ConformanceLevel setting is set to ConformanceLevel.Fragment or ConformanceLevel.Auto if you want to write an XML fragment. (Fault Detail is equal to Tridion.ContentManager.CoreService.Client.CoreServiceFault).

Other Technical Details

Exception : Tridion.TranslationManager.DomainModel.Exceptions.LocalizableArgumentNullException: Value cannot be null.
   at Tridion.TranslationManager.DomainModel.Api.ErrorEventArgs.get_TcmItemTargetUri()
   at ErrorEmailNotifications.ErrorEmailNotificationsPlugin.ReplacePlaceholders(String source, ErrorEventArgs e, Boolean isText)

We use to translate the different type of components (based on different schemas), all works fine, however few components of "General" type schema works fine and few don't.

We are using customized extension called "ErrorEmailNotificationsPlugin" which generates the exception errors for "on Tridion.TranslationManager.DomainModel.Api.ErrorEventArgs.get_TcmItemTargetUri()"

Any suggestions?