FortiOS 6.2.1: FortiAPs won’t connect anymore

Some customers have reported, that their FortiAPs won’t connect anymore after upgrading to FortiOS 6.2.1.
Fortinet has confirmed that this is a know issue only when using trusted hosts to restrict the administrative access to the FortiGate.

The official workaround is to add the FortiAP’s IP or subnet as an additional trusted host entry on one of the admin users:

config system admin 
  edit "adminuser"
    set trusthostx <-- IP Address of the FortiAP