Hi Kristian,
To diagnose the problem more info is needed. It may be possible to get a better error message if you edit the Web.config file,
look for this part:
<customErrors mode="RemoteOnly" ...
and change it to this:
<customErrors mode="Off" ...
it should hoepfully give a better error.
In IIS 7 you should make sure NETWORK SERVICE and IUsr accounts have read permission on the whole folder, and have write permission on the /Data and /App_Data folders.
Hope it helps,
Joe