Hi Thierry, i dont remember how I fixed it. As im currently moving out of my apartment, I dont have my old files with me to check. If you can´t figure it out by yourself let me know in a week and ill look for the old files.
Out of the top of my head: Try placing the load level at the end of the string, after the widgets are destroyed. Make sure that the loading level loads a different widget or none at all.
Im not sure where those BPs are located but maybe you need to connect the ¨Menu_C Widget" as a target for the remove from parent node on the ¨OnClicked (play)¨ srting.
Hope some of these ideas help you while im moving into my new apartment.
Good luck,
Seichi