So – A year has passed, two Ubuntu versions, what’s the state of MTP vs. mass storage? Apparently still broken. Same same but different.
Instead of making the player crash, gphoto2/libmtp and usb mass storage fight for the right to access the device which leads to very ugly errors in dmesg that look like the device or at least its file-system is severely broken 🙁
So, once again: Either make the device MTP only or remove the device’s entry from the libmtp udev rules which now reside in /lib/udev/rules.d/45-libmtp8.rules