I think it's not issue regarding mount (on my Loki, it's mounted), but seems like Pantheon is unable "parse" path to device.
How to reproduce:
1. connect tablet
2. it shows up in Pantheon left list of "Devices"
3. click on it, you get "Unable to mount folder", device change status to mounted
4. go to ls /run/user/1000/gvfs/mtp\:host\=%5Busb%3A001%2C009%5D/Tablet/ # replace mtp\:.... with your device
5. browse content
I think it's not issue regarding mount (on my Loki, it's mounted), but seems like Pantheon is unable "parse" path to device.
How to reproduce: 1000/gvfs/ mtp\:host\ =%5Busb% 3A001%2C009% 5D/Tablet/ # replace mtp\:.... with your device
1. connect tablet
2. it shows up in Pantheon left list of "Devices"
3. click on it, you get "Unable to mount folder", device change status to mounted
4. go to ls /run/user/
5. browse content
So it look to me like parsing problem.