In version 5.4.76847 you may run into the following issue:
Under some browsers (usually Internet Explorer, but also some Firefox and Chrome versions seem to be affected) you try to login using the adminui but you get the error message: username or password wrong.
Currently there seems to be no solution but a workaround:
1. Check for a working browser.
2. set the language settings of the browser to en-us.
3. change the locale setting from default to any language.
Apply the settings and you are able to access the box with any browser.
If all browsers affected set the browser locale to en-us.