Hello @Gareth Parry ,
This is what I was thinking !
When you run the ASP.net application from the visual studio and after performing the authentication - those user details will be stored in SQL Server localdb which gets created automatically where it stores all ASPnetRoles, userClaims , UserLogins , UserRoles etc.
Try to check out your web.config file , it might have some entries like below ?
<add name="DefaultConnection" connectionString="Data Source=(LocalDb)\MSSQLLocalDB;AttachDbFilename=|DataDirectory|\aspnet-20220416092237.mdf;Initial Catalog=aspnet-20220416092237;Integrated Security=True" providerName="System.Data.SqlClient" />-->
Probably , it is complaining about that auto generated localdb SQL Server...
Regards,
Shiva.