I too encountered errors when intially signing up. Make sure you use only one-word usernames. The password is E-mailed to you, but I believe there is no way of changing your password once you receive it. This practically forces the use of the "remember password" option, something I don't typically agree with.
Secondly, the 'formatting' for challenges tends to be a bit poor. You actually have to locate where the page itself is stored. I would expect this for the occasional exploit challenge, but not for every challenge on the site! For example, in Mozilla Firefox you can't even solve the first Javascript challenge without finding the actual page it stores the data on!
I believe it is impossible to complete most HackQuest challenges without Internet Explorer, due to the handling of .php files.
http://www.hackquest.de/modules.php?op=modload&name=HackQuest&file=challengestart&cid=1
...is actually...
http://www.hackquest.de/modules/HackQuest/hacking/394/394.php