I have exactly the same problem on my TS-412...
The version of pl2303.ko I have was compiled under a previous firmware/kernel release, but (surprisingly) doesn't complain when I insmod, and dmesg says all the right things:
[ 2125.020000] usbcore: registered new interface driver usbserial
[ 2125.020000] usbserial: USB Serial Driver core
[ 2136.240000] USB Serial support registered for pl2303
[ 2136.240000] pl2303 1-1.2:1.0: pl2303 converter detected
[ 2136.260000] usb 1-1.2: pl2303 converter now attached to ttyUSB0
[ 2136.260000] pl2303 1-1.3:1.0: pl2303 converter detected
[ 2136.280000] usb 1-1.3: pl2303 converter now attached to ttyUSB1
[ 2136.280000] usbcore: registered new interface driver pl2303
[ 2136.280000] pl2303: Prolific PL2303 USB to serial adaptor driver
However, when I open the USB serial port I have a kernel crash and it reboots the NAS.
I have tried many (possibly all) the threads on this subject with mixed success.
The most recent attempt was to "ipkg install kernel-module-pl2303", but it's not in the repo
This pl2303 chipset is widely used by many people, and I *have* had it working fine under previous kernel versions (when compiled by hand) so I don't understand why QNAP don't include it as a default module. Or at least release the kernel source to give users a fighting chance of building their own.
It would be nice if someone from QNAP could comment on why support for this chipset is proving so hard to provide...
Should have gone for a Synology NAS instead... Never mind.