The Original Prusa MK4/MK3.9 is able to manage several tasks simultaneously: this is called multithreading. In case of an issue related with multithreading, the printer will show the message: "Marlin client could not send message to Marlin server and timeout was reached."
Error name: Marlin Request Timeout
Error code: #13530 (MK4) #21530 (MK3.9)
Restarting the printer is expected to resolve the error, and this is not expected to occur again. In case a restart does not resolve it, please contact our customer support, we will take it from here.