Setup: Login in the selected instance failed.

Setup: Login in the selected instance failed.

  
The installer could not connect to the installed "OUTSYSTEMS" instance of SQL Server. So, using SQL Server Mngmt Std Express, I enabled "mixed Windows / SQL Server authentication.", then enabled the "sa" account and set the password to "outsystems" for this account, typed the correct password into the installer's "Configure SQL Server" step. But it will not connect. ('Wrong username or password'). What have I forgotten?
Problem solved:

In SQL Server 2005 Surface Configuration, go to "Service Area Configuration for Services and Connections", and there, enable "Remote Connections" for the OutSystems database instance, using "TCP/IP" only.

Perhaps remote connections can be disabled after setup has finished, but I haven't tested that yet.