License activation error: Sorry, there was an error, please try again. (The underlying connection was closed: Could not establish trust relationship for the SSL/TLS secure channel.)

Such error could be thrown on the "ManageLicense.aspx" page during licence activation in Central Administration.

To fix this issue you need to install SSL certificate to SharePoint trust relationships. And activate license afterwards.

To do this, follow this steps:

1. Download root certificate for https://license.elearningforce.com  (root_cer.cer - root SSL certificate for EFI licence service)
2. On Central administration server, open SharePoint 2010 Management Shell. Type:

   New-SPTrustedRootAuthority -Name "FriendlyNameForTrust" -Certificate C:\location_of_certificate

3. Ensure, that trust relationship was added at Central Administration -> Security -> General Security -> Manage Trust: 
4. In Central administration server verify that links to CRL (Certificate Revocation List) can be opened:
   http://crl.geotrust.com/crls/gtglobal.crl  
   http://rapidssl-crl.geotrust.com/crls/rapidssl.crl 
5. Try to activate license