Ok, in fact, there is a revelant information in dmesg just before "Unknown symbol" errors :
[ 519.722102] usb 5-1: new high speed USB device using ehci_hcd and address 5
[ 520.028765] usb 5-1: configuration #1 chosen from 1 choice
[ 520.243160] ieee80211_init: failed to initialize WME (err=-17)
[ 520.248363] rt2x00lib: Unknown symbol ieee80211_register_hw
[ 520.248639] rt2x00lib: Unknown symbol ieee80211_tx_status_irqsafe
[ 520.248990] rt2x00lib: Unknown symbol ieee80211_ctstoself_get
[ 520.249118] rt2x00lib: Unknown symbol ieee80211_start_queues
[ 520.249367] rt2x00lib: Unknown symbol ieee80211_stop_queue
...
Ok, in fact, there is a revelant information in dmesg just before "Unknown symbol" errors :
[ 519.722102] usb 5-1: new high speed USB device using ehci_hcd and address 5 register_ hw tx_status_ irqsafe ctstoself_ get start_queues stop_queue
[ 520.028765] usb 5-1: configuration #1 chosen from 1 choice
[ 520.243160] ieee80211_init: failed to initialize WME (err=-17)
[ 520.248363] rt2x00lib: Unknown symbol ieee80211_
[ 520.248639] rt2x00lib: Unknown symbol ieee80211_
[ 520.248990] rt2x00lib: Unknown symbol ieee80211_
[ 520.249118] rt2x00lib: Unknown symbol ieee80211_
[ 520.249367] rt2x00lib: Unknown symbol ieee80211_
...