What a miracle - it runs again. But by the way, if you find the time to have a look at my posts: as I wrote above, I have another Joomla with (a licensed) AcyMailing. I matched both configurations but can't find any different. But there is a different in the choosen URL at status line while mouse over the link. In case of the licensend installation there get shown: mysite.com/linkname. In case of the struggling AcyMailing there get shown myothersite.com/index.php/linkname. Why does this happen or what could be the reason? And, while it not work properly, there get shown myothersite.com/index.php/Categories/linkname. But this seemed to get changed by a wizards hand (or a remote configuration by AcyMailing?).
Anyway, many thanks for notice me.