hi Dirkdigs,
You can call the AP' s GUI through http://ipaddress if enabled.
And kindly update if you get anything useful for trouble shooting. :)
What I understand from the " control message " keyword, the FortiAP' s are using the protocol CAPWAP to maintain the trust relationship with the Controllers.
And if you have a log that " Control message maximal retransmission limit reached" , that means the AP' s are failed to find the Controller.
It might be an issue with the physical connectivity between the FGT and FAP' s. But how it will happen for all AP' s together. I have cross checked the cabling and power , everything is perfect.
And Yes, It seems AP' s are not stable with the version 5.2, earlier I didn' t have this much issues , even though occurred the same few times.
Nihas [\b]