Can it be that modplug is writing to a small harddisc, and the harddisc actually runs out of discspace?
It could be that it goes to C:\temp first where C: has little space left?
That would be my first clue.
Can you test this behavor by exporting a one pattern song (or just one pattern of your song with all the tracks, to see if it still happens?) 1 pattern should illiminate any disk usage problems as the output will be 11x small = still small

EDIT: I have just tested this behavor by rendering a 3 minute lengthy song consistent out of 12 used channels but due to work in progress and lazyness still has 32 channels in total, and it uses VST's and VSTi's. I rendered the track to 32 wave files of each 3 minutes long, resulting in a near 1 GB of space on my harddisc, but no error like mentioning above. Then again, I do have plenty of free space, which could be a trigger for this kind of behavor.
Also if the place you want to render to is on a network location and a glitch occurs, this could happen.
EDIT2: Odd... As I'm finished with the process, I want to remove the directory I created for this purpose. It deletes all files, but it says the directory is in use. Closing MPT does however allow me to remove the directory. Perhaps it has to do something about creating a lock on the directory once a render has started?