Up to now I could finally track down the issue. An excel workbook with VBA password protected project with userforms on it. Whenever I use the command line "Userform1.show" -buttons, or even in the inmediate window-, This VBA password request prompts at closing the workbook.
Other words, the problem does not occur if "Userform1.show" is not executed.
I don´t have any addins active.