OctoPrint Update 14.0 1.4.0rc1 from 1.3.12 '502 Bad Gateway' from Cura and printoid gives this error every 5 seconds or so 'command error:api/login?passive=true(400)'
deleted printer from cura and re added it but knew it wasn't going to help seeing as printoid would error too
Octoprint print fine when you upload the gcode via a web browser though. Just a heads up to anybody who's interested.
I've gone back to 1.3.12 and EVERYTHING is working just like before 
This is a known issue with 1.4.0 RC1 and should be fixed with the next RC
Updated to 1.4.0RC2 Cura sent the gcode without error and printed but the SAME Error 'command error:api/login?passive=true(400)' every few seconds in printoid.
You might try it with the Force Login plugin disabled to see if this is what's causing it.
Will do after this print. I'll update and disable the force login plugin and report back.
OK seems my 1.4.0RC2 does NOT have a forced login plugin, 1.3.12 does have this plugin but 1.3.12 has NO problems connecting.
Somewhere in the Settings is something which allows CORS. You might check that on both as well.