It looks like I am not alone with this issue (getting "Communication timeout - reset send buffer block").
For me, this happens at random places (printing [or should I say TRYING to print] the same GCode file).
I understand what you're saying about not seeing "OK" response from the printer firmware, but I don't know how to debug this issue any further?
I'm running Repetier Host on a Windows-10 (64-bit) machine, and I've disabled the USB sleep in the power settings...
I've cranked the BAUD rate down to 38400, no change:
12:32:16.787 : N2649 G1 X142.71 Y284.684 E0.04666108
12:32:16.874 : ok 2649
12:32:16.874 : N2650 G1 X142.454 Y299.353 E0.73195*95
12:32:16.890 : ok 2650
12:32:16.891 : N2651 G1 X142.451 Y299.552 E0.0099386
12:32:16.954 : ok 2651
12:32:16.954 : N2652 G1 X142.251 Y299.549 E0.00998*82
12:32:16.970 : ok 2652
12:32:16.983 : N2653 G1 X141.777 Y299.54 E0.0236598
12:32:17.214 : ok 2653
12:32:17.214 : N2654 G1 X138.038 Y299.46 E0.18658*100
12:32:17.230 : ok 2654
12:32:17.231 : N2655 G1 X137.57 Y299.471 E0.02336101
12:32:17.696 : T:204.88 /205 B:70.24 /70 B@:21 @:40
12:32:17.848 : ok 2655
12:32:17.848 : N2656 G1 X136.913 Y299.367 E0.03319*82
12:32:17.865 : ok 2656
12:32:17.865 : N2657 G1 X136.891 Y299.361 E0.0011482
12:32:17.897 : ok 2657
12:32:17.897 : N2658 G1 X134.863 Y299.325 E0.10119*94
12:32:17.917 : ok 2658
12:32:17.917 : N2659 G1 X134.898 Y297.294 E0.1013481
12:32:18.294 : ok 2659
12:32:18.295 : N2660 G1 X134.812 Y296.602 E0.03479*93
12:32:18.685 : ok 2660
12:32:18.685 : N2661 G1 X135.128 Y278.606 E0.8979689
12:32:18.701 : ok 2661
12:32:18.701 : N2662 G1 X136.012 Y227.913 E2.52947*86
12:32:18.803 : T:205.25 /205 B:70.24 /70 B@:32 @:40
12:32:19.066 : ok 2662
12:32:19.066 : N2663 G1 X136.039 Y226.434 E0.0738100
12:32:19.385 : ok 2663
12:32:19.385 : N2664 G1 X136.734 Y186.646 E1.98533*94
12:32:59.443 : Communication timeout - reset send buffer block
12:35:29.035 : Serial com error:System.IO.Ports.SerialErrorReceivedEventArgs
12:35:29.953 : Printer reset detected - initializing
Note: On LCD screen (on printer), It showed: X=136.73, Y=186.65.
I'll admit, I don't know it that shows where it IS, or where it's GOING, but that's when it went out to lunch!