Instance-specific or network related error happening while connecting to SQL server

Please check the issue by following these steps:

1. Check the connection string is correct.

<connectionStrings>
<add name="ConnectionString" connectionString="Data Source=***;Initial Catalog=***;User Id=***;Password=***;" providerName="System.Data.SqlClient"/>
</connectionStrings>

2. Please make sure that your asp.net membership tables and roles are created to your remote database if you are using asp.net membership and profile

You can run the aspnet_regsql.exe tools, which is located in the
[drive:]\WINDOES\Microsoft.NET\Framwrok\v4..0.30319

3. Add a <clear/> element as the first element under the connection strings element to ensure no strings are being inherited.

and after add connectin string named LocalSqlServer, which is in the Providers section under AspNetSqlMembershipProvider

<connectionStrings>
<clear />
<add name="ConnectionString" connectionString="Data Source=***;Initial Catalog=***;User Id=***;Password=***;" providerName="System.Data.SqlClient" />
<add name="LocalSqlServer" connectionString="Data Source=***;Initial Catalog=***;User Id=***;Password=***;" providerName="System.Data.SqlClient" />
</connectionStrings>

4. Please check 'defaultProvider' and 'connectionStringName' for membership and profile in web.config.

<membership defaultProvider="SqlProvider">
<providers>
<clear/>
<add name="SqlProvider" 
type="System.Web.Security.SqlMembershipProvider" 
connectionStringName="ConnectionString"
applicationName="SampleApplication"
enablePasswordRetrieval="true"
enablePasswordReset="true"
passwordFormat="Encrypted"
requiresQuestionAndAnswer="true" />
</providers>
</membership>

<profile defaultProvider="SqlProvider">
<providers>
<clear />
<add name="SqlProvider"
type="System.Web.Profile.SqlProfileProvider"
connectionStringName="ConnectionString"
applicationName="SampleApplication"
description="SqlProfileProvider for SampleApplication" />
</providers>

</profile>