by the way i just found following in the logfile:
2019-10-01 12:55:25,838 - octoprint.plugins.pi_support - WARNING - This Raspberry Pi is reporting problems that might lead to bad performance or errors caused by overheating or insufficient power.
!!! UNDERVOLTAGE REPORTED !!! Make sure that the power supply and power cable are capable of supplying enough voltage and current to your Pi.
but i don't think this could be the reason - because once it's working, it will work "forever" in the same setup. nevertheless i just ordered a new power supply
See that wavy line in the temperature graph? That's the reason your print never starts. The printer never reaches a stable target temperature as instructed to, it hovers around it but keeps fluctuating. The blocking M109 heatup command thus never finishes. The printer is stuck in an endless heatup loop.
You might want to do a PID tune.
You definitely also want to fix your undervoltage issue.
Guenda Hi, are you willing to share your octoprint profile for creality cr 20pro, i tried to setup cr20 pro a while ago, but i had some strange problems, if i remember correctly it didnt start printing, and after disconnecting from octoprint and tried to print directly from sd card the z offset was completley messed up. I dont remebmer correctly but Iam sure I had serious problems if i tried to use octoprint with my cr20 pro.