I have exactly the same issue. Upgraded from 1.4 to 1.5 with dpkg on a debian system. Nothing in the config is changed, except that I migrated to tsi1. Not matter what I do, there’s simply no tcp socket open.
Did you try ulimit -n 256789 and then start influxd in that shell environment? This fixed it for me, however, adding the line LimitNOFILE=256789 into influxdb.service does not seem to have any effect… Wonder why…
So, basically, when you start influx via systemd it dies because of too many open files, but that is not detected by systemd. So you think the service is running, but the port is closed. In reality, influxd never makes it through startup.