OK, error found after 3 days of hunting. The hook_menu had 3 pages. 1 for entering parameters. Second for showing the question selection. Third for showing the answers. The error was in the second page which still pointed to an old URL which was meanwhile disabled. Yeah, you could say an "off by one" ;-) The very very strange thing is, that the module didn't show even the first working path and that there was no error of it. Case solved. Thanks to all.

hook_menu() had 3 entries: one for entering parameters, one for showing the question selection, and one for showing the answers.

The error was in the second page, which still pointed to an old (disabled) URL. 
The very very strange thing is that the module didn't show even the first working path and that there was no error of it.