Ok it was my fault. The 4GB-Patch didn't work correctly if a client reconnect. If the station data in driver is deleted but in hostapd not it can happen that the data from driver is greater than the data in hostapd that causes gigawords++. We have to clear last_rx_bytes, last_tx_bytes, acct_input_gigawords and acct_output_gigawords on accounting start. regards gunter