What software are you using to physically Burn to disc? This could actually be part of each problem - the Menu and also why your burner is not being seen by PE.
Sometimes home-burned DVDs just don't trigger DVD players to switch to widescreen play.
Manually turning on widescreen play on your DVD player should do the trick.
I'm afraid both answers did not solve the problem. I uninstalled Nero, restarted the computer but all stayed the same. I really cannot imagine this could also solve the 4:3 menu problem.
Desperate as I was I created a new project and there the problem was over (both of them). Reloading the old project "solved" the burning problem (it suddenly found my DVD burner) but not the 4:3 menu problem. So I installed the second DVD with all the menu's, just selected another one and then the problem was over! Also when I turned back to the original menu. So in some way it has "reset" the menu. Hopefully this can help other persons who have the same problem. Thanks for your answers!!