provider: SQL Network Interfaces, error: 25 - Connection string is not valid

Connection string was like this;

<add name="MyDbConnectionString" connectionString="Server=(svrcis)\MSSQLSERVER; Database=WebRegister; User Id=WebRegister; Password=poker; Trusted_Connection=false;"
         providerName="System.Data.SqlClient"/>

I removed the instance name and worked for me:

<add name="MyDbConnectionString" connectionString="Server=svrcis; Database=WebRegister; User Id=WebRegister; Password=poker; Trusted_Connection=false;"
         providerName="System.Data.SqlClient"/>