Kong pods stopped running - but not sure why [ attempt to index local ‘app_cls’ (a userdata value)]

Can someone please explain the meaning of the exception below, this started occurring in our environments today and a restart of the pods fixed the issue. But I am trying to understand what exactly went wrong with these pods that were running perfectly fine and no changes occurred in the environment.
We are hosted in Azure and kong cluster runs in an AKS cluster .

2019/07/22 19:00:49 [error] 5346#0: *49599051 lua entry thread aborted: runtime error: /usr/local/share/lua/5.1/lapis/init.lua:17: attempt to index local ‘app_cls’ (a userdata value)
stack traceback:
coroutine 0:
/usr/local/share/lua/5.1/lapis/init.lua: in function ‘serve_admin_api’
content_by_lua(nginx-kong.conf:168):2: in function <content_by_lua(nginx-kong.conf:168):1>, client: XXXXX, server: kong_admin, request: “GET / HTTP/1.1”, host: “”
packet_write_wait: Connection to XXXXX port 22: Broken pipe199 “-” “kube-probe/1.12”