I didn’t look in details into your setup. But what I noticed in my non-standard setup using systemd services and a diconnected network is that both seafile and seadoc should be able to see each other at their official DNS host and connection schema (e.g. https).
Also if this involves custom CA and certificates, they need to follow all current standards for AltName and type.
I have some notes here: Seadoc accesses seahub not through SEAHUB_SERVICE_URL - #9 by seadolphin
I didn’t have yet time for a detailed write-up.