enterprise pritunl zero gives 502 with application load balancer port 403

recently we started using pritunl zero enterprise version for our internal services as extra layer of security, so there were already application load balancer listening on port 443 and configured multiple targets based on rules but when i am trying to access service its giving http 502 error, with same configuration if i use load-balancer with port 80 its not happening, i am unable to understand this behaviour.

we are using aws for certificates and load balancer also.
here is simplified configuration we have done which giving 502 error.
internal-service-dns → pritunl-zero → application-loadbalancer:443.