Yesterday at work I had a funny issue with three APs in a portchannel. On the switch a portchannel was created. APs were restarting all the time: they would associate with the WLC and restart after a minute or two. After enabling debug I saw CAPWAP-3-DTLS_CONN_ERR on the WLC for one of those APs.
The solution was to enable LAG for those APs on the WLC with the command:
config ap lag-mode support enable <name of AP>
Cisco really makes it difficult to troubleshoot this issue as the error has nothing to do with LAG. This document, however, makes it very clear how to do LAG for APs: