Cannot connect to local SQL Express instance during Umbraco install

Questions
1

I am trying to install Umbraco using the Web Platform Installer, and it is rejecting my database credentials for my local SQL Server Express instance.

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I’ve gone into SSMS and confirmed that I have the correct credentials for the admin account. I first had to log in using Windows Authentication, then once in, I altered the sa account.

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Does anyone know what I’m doing wrong? Thanks.

EDIT

It must be something wrong with the way I’ve configured things, because even trying to log into SQL Server Express using the sa credentials I just edited does not work.

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This is a companion discussion topic for the original entry at https://our.umbraco.com/forum/79967-cannot-connect-to-local-sql-express-instance-during-umbraco-install