An exception occurred when trying to issue security token: The security token username and password could not be validated. + FBA Sharepoint 2010

This error might be caused due to various reasons

Main reason :Service account has not added to your FBA database.

IIS >Application pools > SharePoint Central Administration v4  > Advanced settings >

under process model > identity > you can see service account  here

2nd reason :  check your web service web.config file, you need to include below code 

Go to IIS Manager>SharePoint Web Services\SecurityTokenserviceapplication

Or

C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\14\WebServices\SecurityToken\web.config 

<system.web>

    <roleManager>

      <providers>

        <add connectionStringName="SQLConnectionString"

            applicationName="/"

            description="Stores and retrieves roles from SQL Server"

            name="SQLRoleProvider" type="System.Web.Security.SqlRoleProvider, System.Web, Version=2.0.3600.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" />

      </providers>

    </roleManager>

    <membership>

      <providers>

        <add connectionStringName="SQLConnectionString"

            passwordAttemptWindow="5"

            enablePasswordRetrieval="false"

            enablePasswordReset="true"

            requiresQuestionAndAnswer="true"

            applicationName="/"

            requiresUniqueEmail="true"

            passwordFormat="Hashed"

            description="Stores and Retrieves membership data from SQL Server"

            name="SQLMembershipProvider"

            type="System.Web.Security.SqlMembershipProvider, System.Web, Version=2.0.3600.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" />

      </providers>

    </membership>

  </system.web>

</configuration>