The Original Prusa MK4/MK3.9 is showing the message: "Emergency stop invoked by G-code (M112)".
Error name: Emergency stop
Error code: #13510 (MK4) #21510 (MK3.9)
This error message can be triggered during serial printing (e.g. Octoprint) and blocks the printer control.
Temporarily disconnect from your printer any external device, reboot the printer, and try printing using a FAT32 formatted USB drive. See if a different error would occur while printing via USB drive, and check the article relative to the new error.