Chris,

To me this is one of most common errors that happen when you use MSSQL and a Web Application.

The error tells me that WebApp runs under the system account (see NT AUTHORITY\SYSTEM).

The error also tells that this account does not have access rights to the used database. That means start SSMS, go to security, logins, find NT AUTHORITY\SYSTEM, open the properties, go to user mapping and allow the database you need to be accessed by this account.

Harm asked if you ran the v24 webapp as a different user as that would explain why a v24 app can access the database and a v25 cannot. The webapp server can run under a different account than system but a webapp on its own can use a different account as well. Anyway v25 uses the system account (according to this error message).