Changepassword Connection String ! how to configure

Hello

I use the default manage.aspx page in a ASP.net application

on my local machine to works with no issue

I uploaded it to a hosting server , and changed the connection string in the web.config

all forms work fine with no issue

but the changepassword do not work and I get an error as below

A network-related or instance-specific error occurred while establishing a connection to SQL Server. The server was not found or was not accessible. Verify that the instance name is correct and that SQL Server is configured to allow remote connections. (provider: SQL Network Interfaces, error: 26 - Error Locating Server/Instance Specified)

I guess it is a connection string issue

but how could I configure it ?

thank you