What is the problem?
With OctoPrint 1.3.10 I'm getting Recv: busy:processing
Printer seems to support the busy protocol, adjusting timeouts and setting busy interval accordingly
and then a lot of Recv: busy:processing
What did you already try to solve it?
Nothing yet, printer is still printing
Additional information about your setup (OctoPrint version, OctoPi version, printer, firmware, octoprint.log, serial.log or output on terminal tab, ...)
OctoPi 0.14, OctoPrint 1.3.10, LulzBot TAZ 6, Marlin 1.1.9.28
Those messages come from your firmware. It's the firmware telling any connected host that it's currently really busy with something (e.g. a blocking heatup, homing or other long running command) and therefore cannot receive and process any new commands for the time being.
However, if you are in the process of troubleshooting or building new printer and temporarily disconnect thermal sensor of tool 0, the temperature value may turn negative and above filter won't apply. The fix is easy - just add '-?' in front of the last \d.