Yes, this can be a pesky issue. The problem usually is caused by some corruption somewhere in the project, although it might be hard to find. But other things like mixed formats and positions of titles can cause the issue. Or it can be project related, that can be cured by copy and pasting into a new project.
Take a look at this thread for some various solutions that have worked for others:
Help with iMovie- Error message 10008
Some solutions that have worked:
1. Create a duplicate of the project and export that.
2. Reposition titles. (Also can try changing the fonts in titles in case of a corrupted font.)
3. Convert your clips to .mov or Mp4 -- no mixed formats among clips.
4. At the point where the movie stops rendering, find that point in the project and remove some frames before and after.
5. Copy and paste the project into a new library.
6. Update both operating system and iMovie version to current. (iMovie 10.1.8 was designed for High Sierra.)
Sorry you are having this trouble. Nothing to do but keep experimenting until you can pinpoint the point of corruption.
-- Rich