We have an application framework which starts all sub applications in its own ui thread. In this case the WPFLocalizeExtension had problems to resolve translation tokens. This was caused by the well known thread access exception. I took the source code
and could find the place which is responsible for the exception.
In the LocExtension.cs in line 407, the VisualTreeHelper tries to walk up the control hierarchy. This could be called from the wrong thread in applications with n ui threads. I changed the code to the following and could solve the problem.
doParent2 = VisualTreeHelper.GetParent(doParent);
DependencyObject parent = doParent;
Action update = () =>
doParent2 = VisualTreeHelper.GetParent(parent);
Maybe this little change can be integrated in the near future.