The "lpcm" error has happened to others, and not just with High Sierra. It seems to have something to do with the audio, as "lpcm" refers to an audio format. I am not aware yet of any global solution. See this thread for a discussion, and you might try some of the things mentioned in it:
iMovie 10.1.7 Export Failed
If you haven't updated to iMovie 10.1.8 yet, do so because that version was designed for High Sierra.
As you have found, exporting at pro res has been a reliable workaround, with some people afterwards converting the shared movie to Mp4 or .mov if they didn't want the large file size. Open it in Quicktime Player and then save it. You will get a .mov file. Or you can convert it with the free download, Handbrake, that by default will provide you with an Mp4/AAC file that iMovie likes.
You also can try importing your original clips to your desktop with the image capture app on your Mac instead of importing directly to iMovie. Import those to iMovie and see if they share properly. If not, before importing to iMovie open and save them to a .mov file with Quicktime Player and then import them into iMovie and see if you can share out properly with them at other than pro res.
Check the audio codec of your clips and make sure that they are AAC or aiff. If not, do Handbrake conversion to get an Mp4/AAC .
You might try some of the standard procedures, such as:
1. Opening iMovie while holding down the option and command keys, and then selecting to delete preferences in the box that appears.
2. Creating a new project and importing your clips into the new one, and then see if you can export properly.
3. Opening iMovie in a new user account, such as the Guest account on your Mac, and then seeing if iMovie exports correctly.
Hope this helps.