Sitecore 7: Troubleshooting: LDAP/AD: non-existing provider referenced by the “providername”

Problem:
I have installed the official LDAP / AD sitecore package. Now you need to make some changes in the web.config to make it running.
Well this error appears in my case with a non proper LDAP/AD configuration in the Web.config file.

Solution:

In my case the provider tag “ad” was missing in the role Manager section.

   1: <switchingProviders>

   2:      <membership>

   3:        <provider providerName="sql" storeFullNames="true" wildcard="%" domains="*"/>

   4:        <!-- LDAP1 -->

   5:        <provider providerName="ad" storeFullNames="false" wildcard="*" domains="ad" />

   6:      </membership>

   7:      <roleManager>

   8:        <provider providerName="sql" storeFullNames="true" wildcard="%" domains="*"/>

   9:        <!-- LDAP1 -->

  10:        <provider providerName="ad" storeFullNames="false" wildcard="*" domains="ad" />

  11:      </roleManager>

  12:      <profile>

  13:        <provider providerName="ad" storeFullNames="false" wildcard="*" domains="ad" />

  14:        <provider providerName="sql" storeFullNames="true" wildcard="%" domains="*"/>

  15:      </profile>

  16:    </switchingProviders>

Also add tis here after the closing membership tag.

   1: <roleManager defaultProvider="sitecore" enabled="true">

   2:       <providers>

   3:         <clear/>

   4:         <add name="sitecore" type="Sitecore.Security.SitecoreRoleProvider, Sitecore.Kernel" realProviderName="switcher" raiseEvents="true"/>

   5:         <add name="sql" type="System.Web.Security.SqlRoleProvider" connectionStringName="core" applicationName="sitecore"/>        

   6:         <!-- LDAP -->

   7:         <add name="switcher" type="Sitecore.Security.SwitchingRoleProvider, Sitecore.Kernel" applicationName="sitecore" mappings="switchingProviders/roleManager" />

   8:  

   9:         <add name="ad"

  10:             type="LightLDAP.SitecoreADRoleProvider"

  11:             connectionStringName="ldapConn"

  12:             applicationName="sitecore"

  13:             username="<username>"

  14:             password="<password>"

  15:             attributeMapUsername="sAMAccountName" />

  16:         <!-- LDAP -->

  17:         

  18:       </providers>

  19:     </roleManager>