As Alexey said, the issue could be with code, not ECP (as usually ECP is backward compatible)
Make sure that code on app. servers in not mapped (no routine, package mapping) to a remote DB (on database server) - all code should be local to app. server

If your 3 app. servers are identical (for HA, LB reasons) then if you can disable access to 1 app. server and still working with other 2 - it will give you an option to to the app. servers one by one without downtime.

Another issue to concern is cached queries - on such upgrades its good to have all cached queries "un-freeze" to make sure all are compiled.
 

You can call directly to: Do BACK^%ETN it will save the $ZE + the stack. (LOG is also calling this one)

add a debug to OnPreHTTP and check the ^%global
k ^%global S a="" F  s a=$O(%request.Data("")) Q:a=""  m ^%global(a)=%request.Data(a)

sometimes, better to check: $D(%request.Data("action",1)) and not $G