Works now!
Look here:

Hello,

as I want to save the preferences for user_tweaks (enable additional comment permissons), I get this:

Fatal error: Call to a member function forceMenuRebuild() on a non-object in /mnt/web5/d0/95/5201395/htdocs/drupal-tutorial/sites/all/modules/user_tweaks/user_tweaks_ui.module on line 90

After go back, the message from drupal is:
Notice: Undefined variable: var in user_tweaks_ui_config_submit() (Zeile 90 von /mnt/web5/d0/95/5201395/htdocs/drupal-tutorial/sites/all/modules/user_tweaks/user_tweaks_ui.module).

Any idea?

Thanks,
Stefan