I know this has been covered quite a bit here on the forums, but i just bumped into the dreaded ‘MakeOTF’ error.
Luckily I had been saving versions, so I went back to the last working one, then, using the next saved version, deleted single characters until i got it working again. Once i found the character causing the problem, I noticed that I had a tiny path segment exactly overlapping a separate closed path. I deleted the little redundant segment, and the MakeOTF error went away.
Hope this helps someone.