render stop when RFW is open

[HHbomb]

hello
I notice several times that when I render with Rendered Frame Window checked, a few frames rendered and then progress bar stop, and freeze.
Slave computers no more render, progress bar is frozen.
Octane 2.1 maxdesign 2014 with net render.
Same thing when backburner rendering, when a new job in the list is launched, few frames and then freeze.