After installing http://support.microsoft.com/KB/3008923, known issue #1. there might be a problem creating inbound and outbound attribute flows in any Synchronization rule. IE11 have some issues here ...
To solve this, just install Windows Patch 3025390. This Patch does not require any restart either on Windows 8.1 or Server 2012 R2.
I found a thread around this matter on: https://social.technet.microsoft.com/Forums/ru-RU/2d72a287-4fa4-439b-8066-45ff47f2ed4d/synchronization-rules-cant-create-outbound-attribute-flow?forum=ilm2
With this post I hope that you guys find it easier to find!
Regards
//Jörgen