jpdumon The problem is in most cases caused by an active path change in a focus trigger. For example when you add a $prompt in a field gets focus trigger to another field than the current field. The message comes from the UNIFACE kernel, in your case it is possible that Windows does a focus request and in a form loses focus or a field loses focus trigger there is a prompt function. Or as I saw recently a deleteinstance of a form requesting focus, the parent instance losing focus had the deleteinstance in the form loses focus. In all cases avoid any active path changes in focus triggers, also do not start a messagebox with message/info in a focus trigger since windows may steal away the focus from UNIFACE in such a way that the kernel loses track. Success with finding the issue Jasper de Keijzer

Thank you Jasper I will take a look around this !