Hi all
My understanding is that the grunt task concatenates all the various *.less files together into adapt.less before then attempting to compile it to css. So you shouldn't ever need to edit adapt.less
Given that it does work if you 'force' it, I'd suggest it's actually something going wrong in the grunt process rather than anything you've done.
What's strange is that, even though it's not quite the same error, this particular line of LESS does seem to be at the heart of quite a few reported problems, specifically here, here, here and here.
The grunt build tool is actually being changed fairly extensively at the moment so this update may solve this issue. If you have any time to test the update, that would be very helpful.