hey!
i fixed the issue on my own. after 2 days finally. somehow during the switch from MU to MS the database got a bit confused. the mainsite was referencing to the wrong db-prefix (WP_1_ instead of WP_). i have now copied the files over to the WP_ tables and now everything is sorted.
So, Main-Site contents reside inside tables with WP_ prefix (as long you have defined WP_ during the setup) and the stuff from your subsites reside in dynamically created tables with WP_2_ , WP_3_ , WP_4_ …. and so on (2, 3, 4 references your blog id).
HTH for some people with the same problem.
Best regards
zmb