Hi, i have the same problem with an R3 NAS, 4.01 version. I can't access to the Web GUI, from both web browser (IE6, 7 or Firefox).
The first page was correctly showed but there isn't the login form (username and password).
Now, i try to replace the DOM with another (version 5.0) and i would to know if there should be problems with my data after replacing.
You will not lose your data by replacing modules and you can use the Demo-CD as well to keep going in case you need to RMA. I always recommend to save your settings from GUI.
After trying every possible option (direct connect, different browsers, accessing from different systems like Windows and Linux) I decided to take an older version of the software, which had already proven to work in the past and installed that from CD.
Now I can access the WebGUI again and manage the NAS, but I'm back to version 4.21.NE00000000.2745
Still baffeled about the cause of all the problems. Hopefully a solution will come in a future update.