Hi,
Exact same problem here (on windows for me and I don’t even see file with PTP)… It was still working yesterday, I didn’t change anything and now I got empty DCIM file which is really frustrating because it’s impossible for me now to back up my trip pictures T.T
Enabled File transfert and USB Debugging Mode without success :’( Can’t find any solution about this persisting pb … Anyone can Help?!
Thanks…
Edit: I finally found a solution to my pb : (which was) switching to MTP with USB debugging ON and still DCIM folder showed empty.
THE solution:
Go to Settings > Apps > (touch the 3 point in the up-right corner) Show System processes (or something like that, I translate from french) > Media Storage > Storage* > Clear DATA.
Don’t be afraid, it has something to do with the way that your fp organizes the data (so it doesn’t affect your data, just the way your phone sees it).
NOTE: It is important to let your fp rebuild his ‘library’ so wait at least 10min and check if you can load properly your pics on the phone (like usual), if it takes to much time, there’s a free app that helps the phone to rebuild it properly (so the phone can show you everything it should) : it’s called media rescan. I did it just to be sure around 20min after I cleared the data of Media Storage.
Then connect your phone like your were used to, choose MTP and it works ! 
Finally a non-empty DCIM folder : that feels so great 
*Merci pour la remarque Denis 