Hi,
oh man I sweat like crazy. For hours my server was unusable. But I fixed it for me now. I connected to the database and removed the external shares:
[mysql] delete from oc_share_external;
I checked before (with select * from oc_share_external;) and only had the two test shares from the Federation Sharing in that table. After I deleted them I checked the other tables that have anything to do with external and federation, but they contained no other data which relates to the two deleted entries.
Now my server is super fast again! 
