I have just had the same problem with my LS-CHL NAS (firmware v1.07). I have been using my own password to access the web console since I purchased the device (late 2009) then suddenly after a power outage it stopped working. I followed the instructions to reset the device to factory settings however the default password also would not work...
Unlike most people however today the device seems to have corrected itself. I think the problem is related to the time, when the device was not working properly I was still able to login with the guest account and it showed the time to be some date in 2007 (I assume whenever that version of the firmware was created). Now it is working again it is showing the correct date.
If anyone else has the same issue try the following:
1 - log into the web console using the guest account (no password)
2 - make a note of the time displayed
3 - reset the clock on your pc to match that time
4 - try logging back into the web console with whatever the admin password is currently set to
5 - if it works manually set the time on the device to the current date and then correct the time on the PC also
Now as I say my device has somehow corrected the time itself and so I haven't tested this method but it may be worth a go, good luck!