I experienced this problem during my initial setup process when I first bought the router back in December 2012. I have since upgraded to the latest firmware and it just happened again.
I rebooted my router tonight (no changes to the configuration, just a reboot initiated from the router’s admin page), and could not subsequently log in. I tried the default password (“password”) and it allowed me in, so it had reset the password to the default again.
I was able to change it back to my secure password but this bug is definitely not corrected in the most recent firmware which I upgraded to a couple of weeks ago.
Firmware Version: 1.06.18
Firmware Build Date: Fri 21 Dec 2012