Comment 22 for bug 1314556

I have the same problem. But I am able to get it to mount after plugging the phone in, going to Settings -> Developer Options and turning off USB Debuggling and turning it back on again ("Hello IT, have you tried turning it off and on again?").

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agentlogic@Erebus:~$ mtp-detect
Unable to open ~/.mtpz-data for reading, MTPZ disabled.libmtp version: 1.1.6

Listing raw device(s)
Device 0 (VID=04e8 and PID=6860) is a Samsung Galaxy models (MTP).
   Found 1 device(s):
   Samsung: Galaxy models (MTP) (04e8:6860) @ bus 3, dev 4
Attempting to connect device(s)
PTP_ERROR_IO: failed to open session, trying again after resetting USB interface
LIBMTP libusb: Attempt to reset device
LIBMTP PANIC: failed to open session on second attempt
Unable to open raw device 0
OK.