vSZ was upgraded to 184.108.40.206.699 but the problem persists. It might be that as per the tech note you referenced, the hot upgrade might not have been applied. The vSZ in this case, is hosted by a third-party provider, so I can't be sure.
The APs are now at v220.127.116.11.5010. The tech note says the APs should be moved to the 5.2 zone AFTER the hot upgrade.. will this now cause issues if the hot upgrade hasn't been applied. I'm assuming the APs should be at 18.104.22.168.5030? (although the reference links in that patch don't mention IoT, or the R750)
Thanks for any further advice..