I get this error when I try to
render a FLV file. Comp size is 720x1280 (portrait) square pixels and 24 fps.
Using Windows 7 64-bit and Adobe AE CS4 (32-bit)
What is odd is that if I open Adobe media Encoder and drop in the same comp with the same settings, it works. I just should not have to do the extra steps in my opinion.
I remember talk of a bug, where FLV output would not work beyond 1080 pixels in either dimension. This should have been fixed in the 9.0.2 update, but then again it may not or be only half fixed (in one dimension, that is). If so, your current workflow would be the only way to circumvent the issue...