I have old cameras that connect to Linux, like the Canon 350d (older than the 400D) and also a NIkon D5000. I'm sure it's something simple like:
- dodgy cable,
- flat battery,
The PMP setting shouldn't make any difference. Maybe it could get locked if you have automount on, but lsusb should still work.
Does it work at all, even without a USB cable?
try dmesg and see if there are any helpful messages.