Maybe his datastore is messed up when he moved the database?

Ah, that could be it. In which case go to edit a Forum and just click Save. It should rebuild the forum datastore.

I never touched the Config File - except editing the the DB Login Credentials. Also not on the Main site, so I guess there is no xcache in use at all? Just checked my config File and it says:
// $config['Datastore']['class'] = 'vB_Datastore_Filecache';

The Problem wth Export + Import the Database again is that I already modified a lot of stuff on the Test Site, which of course I dont want to loose :/ I was hoping to fix this issue without deleting anything, so I dont delay my hired Designers work :)

What do you mean with "clear your server cache" ? If you tell me, I can let my Hoster do it (my UNIX knowledge is limited)

2 2 1

Just did that... with all Forums! :D But it didnt work....

Thanks for all your help so far :)

try this, in forumhome_forumbit_level2_post template find and replace with now check your forum index page and see how many forum id's you see.

Attachment 140137
Only the Parent Forums show their IDs this way.

Yes that was the idea, this way you can check that the forums show id's are the correct ones for the sub forums that should have been shown.

--------------- Added [DATE]1343386930[/DATE] at [TIME]1343386930[/TIME] ---------------

In forumhome_subforumbit_post look for $forum[title]</a> and replace it with $forum[title]</a> - $forum[forumid] so now any sub forums will show their forumid