Tracks synced, but won't export to USB stick

So all my tracks on Dropbox are also local on my hard drive. But the Dropbox folder structure has somehow changed. I can still play the tracks in my Rekordbox, but as soon as I export them to my USB stick it says that the files were not found.

I changed the folder settings here:

And I went to "File" > "Display all missing files", but it doesn't find any. So within Rekordbox everything looks just fine, only the export doesn't work.

Any ideas where to start?