I ran into this issue with 3800 AP when trying to join it to Mobility Express (ME) WLC over VPN. AP was one minor version off and was showing up on the ME but with an un-joint state. It also had this error message.
AP having MAC Address [“***”] currently Not Joined with this Controller!!!
For clues check under Management > Software Upgrade > Predownload image status. I found my new AP there with the following error.
Server Not reachable
For new AP to join, ME controller must have configuration for TFTP server, point to the folder that contains unzipped software and TFTP server has to be up. ME firmware will have needed images for all supported types of APs.
In my case, TFTP server was down so AP never upgraded and fully joined the ME controller.