I have the same problem, apm.service appears to start when I query with "sudo systemctl status apm.service"
however in the copter.log file it reports a "bad argument" when apm.sh calls Arducopter.elf.
Also all the LEDs on my PXFmini are all on (yellow, orange, blue), so that implies that APM has not infact launched.
Wifi connection from my computer is haphazard, I can putty in sometimes, but not always and then when I do manage to make the connection after some time it loses the connection. (yes, it's the correct ip address everytime, I check my router "connected devices" list first)
Really difficult to use this PXFmini "headless" to diagnose when the connection is so flaky.