Unable to log in to GUI

NAS-R3, latest available firmware. After opening the web GUI, I enter the password and... return to the login form again. When I enter wrong password, I get a warning po-up. If I enter the correct password, I return to the login screen again. Tried IE8, Firefox 3.5.3 and Firefox 2.0 - same result.

Yesterday I could log in without any problem. Since then, nothing was done to the system.

I would not like to restart the whole system since it is a production one. Is there a way to somehow restart the web GUI?

Thanks.

Try clearing cache on your browser

Clearing the cache does not help. However, sometimes I can log in (it seems completely random), but then most of the pages are blank.

Does this happen to all other systems with Firefox?

Not sure what you mean by 'all other systems'... I finally had to reboot and it works again.