I've been trying to think through what might be causing this error.
My site is set up for directories - not sub-domains - would that make a difference?
The plugin in network activated - does that make a difference?
I have tried changing the permalinks (in Settings -> Permalinks) on both sites and it doesn't seem to make a difference.
Are there any known conflicts with other plugins? If it was a conflict with another plugin, would it be one that is network activated or one that is activated on just one of the sites I'm working with?
Am keen to thrash this out and see what could be the error since the developer (@edward_plainview) is not able to repeat the error on his system.
Another thought - Is there any other settings in PHP (or Apache or anything else server related) that could be effecting this?