What kind of communication error is this: (no replies)

now and then and print is destroyed by serial fault
OctoPi may say : Offline (Error: SerialException: 'device reports readiness to read but returned no data (device disconnected or multiple access on port?)' @ comm.py:_readline:2417)

or I see this:
Recv: FIRMWARE_NAME:Marlin V1; Sprinter/grbl mashup for gen6 FIRMWARE_URL:[www.mendel-parts.com] PROTOCOL_VERSION:1.0 MACHINE_TYPE:ok
Changing monitoring state from "Operational" to "Printing"
Communication timeout while printing, trying to trigger response from printer. Configure long running commands or increase communication timeout if that happens regularly on specific commands or long moves.

- what may cause problems like this - it may work fine for a few hours, then destroy a print. - Rebooted octopi & the printer.