raspbian Running but not functional when system boots

[atomicbird]

I'm running Resilio on a new Raspbian install. It works when I start it, but after a reboot it runs but doesn't seem to do anything.

When the system starts up, ps shows that Resilio is running with the following command:

/usr/bin/rslsync --config /etc/resilio-sync/config.json

However syncing doesn't work and the web GUI doesn't respond (and nc shows connections refused for web GUI). It sure looks like it should be running but nothing happens:

$ sudo systemctl status resilio-sync
● resilio-sync.service - Resilio Sync service
   Loaded: loaded (/lib/systemd/system/resilio-sync.service; enabled; vendor preset: enabled)
   Active: active (running) since Sat 2021-05-22 00:36:31 BST; 1min 13s ago
     Docs: https://help.resilio.com
  Process: 379 ExecStartPre=/bin/mkdir -p ${SYNC_RUN_DIR} ${SYNC_LIB_DIR} (code=exited, status=0/SUCCESS)
  Process: 384 ExecStartPre=/bin/chown -R ${SYNC_USER}:${SYNC_GROUP} ${SYNC_RUN_DIR} ${SYNC_LIB_DIR} (code=exited, status=0/SUCCESS)
  Process: 389 ExecStart=/usr/bin/rslsync --config ${SYNC_CONF_DIR}/config.json (code=exited, status=0/SUCCESS)
  Process: 403 ExecStartPost=/bin/sleep 1 (code=exited, status=0/SUCCESS)
 Main PID: 402 (rslsync)
    Tasks: 17 (limit: 725)
   CGroup: /system.slice/resilio-sync.service
           └─402 /usr/bin/rslsync --config /etc/resilio-sync/config.json

May 22 00:36:28 zeroone systemd[1]: Starting Resilio Sync service...
May 22 00:36:31 zeroone systemd[1]: Started Resilio Sync service.

If I restart Resilio with 

sudo systemctl restart resilio-sync

...then everything's fine. What's going on at startup, and how can I get it to start normally? Thanks.