Hi, all.
If this thread is still open, here a solution I’ve found:
edit the following files using the text editor of your preference:
client.py
master.py
operators.py
slave.py
utils.py
they are in \2.56\scripts\io
etrender
replace all occurencies of exr by jpg (or bmp).
save the files, reset the farm.
it worked for me. Now my farm renders in jpg and no black render result occurs.