Then why do those converters work the first time I plug them in but fail after the 2nd time ?
I did remove libmtp-runtime a while ago because it was causing trouble with an Android device, blocking the possibility to mount as mass storage. I just added it back, but no change:
kernel: [155131.056138] usb 5-3: new full-speed USB device number 9 using ohci_hcd
kernel: [155131.224425] pl2303 5-3:1.0: pl2303 converter detected
mtp-probe: checking bus 5, device 9: "/sys/devices/pci0000:00/0000:00:13.0/usb5/5-3"
kernel: [155131.246572] usb 5-3: pl2303 converter now attached to ttyUSB0
mtp-probe: bus: 5, device: 9 was not an MTP device
kernel: [155131.317526] pl2303 ttyUSB0: pl2303 converter now disconnected from ttyUSB0
kernel: [155131.317547] pl2303 5-3:1.0: device disconnected
kernel: [155131.840623] usb 5-3: usbfs: process 4191 (EMT-2) did not claim interface 0 before use