Media Encoder CS5 fails every time on 2 pass VBR?

Hi,
I have tried encoding several videos at multiple bitrates, including videos I have encoded successfully before in CS4.
They all fail about half way through the first pass when using VBR 2 pass. Just stops and shows the error exclamation mark symbol.
I have no problem if it is set to one pass.
I get this totally NOT useful error in the log, any ideas, anyone else had problems??????

- Source File: /Users/gasonmark/Desktop/6-9-2011 from marco h264 for f4v conversion/HoosierTPIR.mov
- Output File: /Users/gasonmark/Desktop/6-9-2011 from marco h264 for f4v conversion/HoosierTPIR.f4v
- Preset Used: Custom
- Video: 512x289, Same as source fps, Progressive
- Audio: AAC, 96 kbps, 44.1 kHz, Mono
- Bitrate: VBR, 2 Pass, Target 0.30 Mbps, Max 0.60 Mbps
- Encoding Time: 00:00:28
Mon Jun 20 01:07:48 2011 : Encoding Failed
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Error compiling movie.

Unknown error.
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