With a WISM, you have by definition an etherchannel between your controller & APs. Try setting the load balancing algorithm on the switch housing the WISM to src-dst-mac before connecting the AP. There is/was an issue that shows itself rarely where other load balancing mechanisms interfere with an AP's initial join to a controller. The problem declined significantly over the years, but you may be running into it.
Once the AP joins, you can reset your load balancing and you shouldn't have the problem even if the AP reboots later. It usually impacts APs that have never joined your controller. Don't know why. Since this was an upgrade it may not be your problem, but it won't hurt to try.
Transferring Crash file from standby: Login to the Active WLC in HA.
From CLI: (Cisco Controller) >transfer upload datatype crash (Cisco
Controller) >transfer upload filename (Cisco
Controller) >transfer upload mode tftp (Cisco Controller) >transfer
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