Weird Sitecore connection string error using SIM

Today I was installing a new instance of Sitecore 8.1 update 3 on my local machine using the SIM tool. Everything went fine until I hot the login screen. Got greeted with below exception

Format of initialization string error
Format of initialization string error
I have to mention that I manually updated my connection strings from an other working Sitecore instance

already existing instance
<add name="core" connectionString="user id=sclogin;password=XXXX;Data Source=(local)\SQLEXPRESS;Database=sc813Sitecore_Core"/>

new instance
<add name="core" connectionString="user id=sclogin;password=XXXX;Data Source=(local)\SQLEXPRESS;Database=elisonSitecore_Core"/>

Weird that both connections strings look exactly same with just database name change.
I tried to reason out why? but then was rushing with setup for a PoC

I ended up modifying the new instance connection string to below and it started working :(
<add name="core" connectionString="Data Source=.\SQLExpress;Initial Catalog=elisonSitecore_core;User ID=sclogin;Password=XXX" />

Sitecore gurus why this could happen?

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