portainer: Unable to access associated Edge Agent from Portainer Web UI

Bug description When attempting to access an associated Edge Agent on a Synology host, the action fails and the error message:

Endpoint is unreachable and there is no snapshot available for offline browsing.

is displayed.

Expected behavior Successfully connect to Edge Agent

Steps to reproduce the issue:

1. Follow instructions in the Edge Agent setup guide (https://downloads.portainer.io/edge_agent_guide.pdf)
2. Create Edge Agent on Synology host using command: sudo docker run -d -v /var/run/docker.sock:/var/run/docker.sock -v /volume1/@docker/volumes:/var/lib/docker/volumes -v /:/host --restart always -e EDGE=1 -e EDGE_ID=<EDGE_ID> -e CAP_HOST_MANAGEMENT=1 -p 8000:80 -v portainer_agent_data:/data --name portainer_edge_agent portainer/agent
3. Associate Edge Agent with Portainer through web UI (Portainer UI states that the agent is associated)
4. Attempt to connect to Edge Agent, which fails
5. Logs for Edge Agent container state that connection was successful (and states fingerprint).

Technical details:

• Portainer version: 1.22.0
• Docker version (managed by Portainer): On Portainer host - 19.03.1, on Synology - 18.09.6
• Platform (windows/linux): Linux
• Command used to start Portainer (docker run -p 9000:9000 portainer/portainer): docker run --restart=always --name portainer -d -p 8000:8000 -p 9000:9000 -v /var/run/docker.sock:/var/run/docker.sock -v portainer_data:/data -v /etc/portainer:/etc/portainer:ro portainer/portainer --ssl --sslcert /etc/portainer/certs/<certname>.crt --sslkey /etc/portainer/certs/<certname>.key
• Browser: Safari and Chrome

Additional context N/a