HI Fabian,
I have this problem with 2 webpages 1 have this ajax tree problem after the up date and the other i can't update because if i do this will stop working.
The issue is with ajax permissions, i think this in the WP update a security fix will have blocked permission, i would think that would have been the case before but not sure where in wp-filebase the fix is, and now with the new update the fix for permissions will be out of date.
Don't know enough to fix this, need an update please.
herebydesign you could put in a short code to show all the files until fixed if that would look ok on your website.