I am having the same problem but the "fix" didn't work for me. I have been able to reproduce this problem with a completely "virgin" recipe - here's what I've done:
1. Install DNN 4.5.5 Starter Kit
2. Open visual Web Developer & create a new DNN Application framework
3. Compile, open site in browser, create default site
(so now I have a completely "green" DNN 4.5.5. install)
4. Open SQL Server Management Studio & connect to the SQLEXPRESS instance on my machine
5. Attach to the database.mdf in the APP_DATA directory
6. Go to browser & refresh home page of the brand new DNN site
7. Site now reports "Cannot open user default database. Login failed. Login failed for user '[machinename]\ASPNET'."
8. Exit SQL Server Management Studio & Visual Web Developer (to ensure they aren't holding db open)
9. Restart IIS
10. Restart SQLEXPRESS service
No good. Nothing I do makes the site come back.
Please advise.