Android File Transfer not working between El Capitan and Android 6
After struggling a lot and checking many solutions off the internet, my issue turned out to be related to "turning off and on again" the SIP (System Integrity Protection) on my Mac. I did the following and it magically works again:


csrutil disable (and then reboot) → ADB/Android File Transfer not working yet.
csrutil enable (and then reboot) → ADB/Android File Transfer already working up to this point.
csrutil enable --without debug → I've gotta do this because I'm using XtraFinder and want to make it work for El Capitan.


See if this resolves yours :)