Octoprint 1.5.3
Marlin 1.1.8
RPi 3
I too suddenly started getting this error on my Octoprint server. I hypothysized the possible error point and found a "fix".
I can configure my printer to use a variety of print heads, some are two-extruder and some are single. So I configured my Octoprint profile to have two heads. In this case, Marlin was configured for one head. I have had this setup for years with no problems between the profiles.
On accident, during one session I pre-heated using tool2 on a Marlin config that had only one head. From that point on, all of my print sessions got the "Not queuings T0" error.
I then re-booted the RPi, and re-connected. Now I no longer get this error. I suppose that Octoprint assumed, because I told it to heat tool 1, that it should have two heads and it started issuing commands to that assumption. Before, all my gcode files had occasional T0 references in M104 or M109, Marlin didn't care. Then after my mistaken front panel fumble, Octoprint got out of sync with marlin and started giving this error.
These are my assumptions based upon the actions of the system and what I did to stop the error.
YMMV,
DLC