The following blog explains the problem:

http://blogs.msdn.com/b/emeadcrmsupport/archive/2013/12/18/dynamics-crm-2011-save-button-on-email-form-is-disabled-once-activity-status-field-is-added.aspx

I have opened a support case with MS on this, but they haven't seen this as a bug. The support case Scope is to remove the Activity status Field on form (Field is system locked).
When the field was removed and form published, the save button was available.

I have done nothing with the ribbon buttons in customization, and JavaScript was also disabled to check for problem.