Login failed for user 'NT AUTHORITY\NETWORK SERVICE'

The error message you are receiving is telling you that the application failed to connect to the sqlexpress db, and not sql server. I will just change the name of the db in sql server and then update the connectionstring accordingly and try it again.

Your error message states the following:

Cannot open database "Phaeton.mdf" requested by the login. The login failed.

It looks to me you are still trying to connect to the file based database, the name "Phaeton.mdf" does not match with your new sql database name "Phaeton".

Hope this helps.

I was experiencing a similar error message that I noticed in the Windows Event Viewer that read:

Login failed for user 'NT AUTHORITY\NETWORK SERVICE'. Reason: Failed to open the explicitly specified database. [CLIENT: local machine]

The solution that resolved my problem was:

1. Login to SqlExpress via SQL Server Management Studio
2. Go to the "Security" directory of the database
3. Right-click the Users directory
4. Select "New User..."
5. Add 'NT AUTHORITY\NETWORK SERVICE' as a new user
6. In the Data Role Membership area, select db_owner
7. Click OK

Here's a screenshot of the above:

If the error message is just

"Login failed for user 'NT AUTHORITY\NETWORK SERVICE'.", then grant the login permission for 'NT AUTHORITY\NETWORK SERVICE'

by using

"sp_grantlogin 'NT AUTHORITY\NETWORK SERVICE'" 

else if the error message is like

"Cannot open database "Phaeton.mdf" requested by the login. The login failed. Login failed for user 'NT AUTHORITY\NETWORK SERVICE'."

try using

"EXEC sp_grantdbaccess 'NT AUTHORITY\NETWORK SERVICE'" 

under your "Phaeton" database.