Regarding "de-registration VNS[networkname] Cause[User changing subnet]" - it's in the station events log right - you get that message e.g. if a MU changes from one SSID to another = subnet change.
Are all APs in one building/area?
Is there only one topology/VLAN used for that SSID or more ?
What topology mode is used e.g. bridge@EWC, bridge@AP, routed?
On the same AP does one client work and another doesn't ?
Check one MU/client with the problem and make sure to delete all known WLANs and try to connect again.
You should also upgrade to the most current sw because 10.41.01 is from November 2017.
-Ron