iPhoto - Faces hidden behind cork layer?

Issue: managed iPhoto library contains about 200 faces and 30,000 items; faces feature displays white frames with assigned names on the bottom for all faces, but no pictures, pictures are displayed as cork (like the background). Following links shows all photos connected with a name. Making new smart albums with FACE → IS results in albums including all photos with that name tag.

And interestingly, when scrolling the 200 faces on the cork board quickly up or down 80% of the faces appear as long as scrolling is continued, but become cork as soon as scrolling stops (mouse/touch pad scrolling has no effect - to slow? - only using the scroll bar makes pictures appearing).

Is there an entry in the xml files or the database that organizes layer sequence and puts the cork-layer for what reason ever on top of the thumbs/pictures?

Tried every rebuild option without any improvement.

Made iPhoto to rebuild deleted faceDB. Took 7 hrs and lots of "found faces(?)" popped up during the rebuild. But the result was Faces coming up with the yellow postit "To start with faces ..."

Any ideas?