5.1 Multiple Mono Bounce to disc problem

[glennloopez]

I have no problem bouncing to disc any of my 5.1 projects before today with PT 8 HD CS2, but today I just got a mono file that needed to be EQ'ed, de ess'd and so on.. so I did and routed it to the 5.1 output with sound only in C but with a limiter on the 5.1 out bus. I even tried with 2 different projects with different set of plugins both resulting in the same: error -6004

ofcourse I use the same bounce settings as always, 5.1 REC out and 16bit 48 khz with Multiple Mono.

is there any good explanation for this? I bounce trouble free old and newer projects but of course the difference is they all have sound in all speakers.