Hi,
This issue has come up recently with several other posters.
I am not sure, but I think that the error means that the transaction timed out due to some conflict, incompatibility or corruption in the project. My Google search did not come up with much about it. Sometimes just shutting down and rebooting cures the conflict.
To rule out a corrupt preference, try opening iMovie while holding down the Option and Command keys and selecting to delete preferences in the box that appears. iMovie will open in a new library. Reopen your old library to get back to your projects. Now see if you can share out.
To rule out corruption related to this specific project, try doing an Edit/Select All, Edit/Copy, of the media in the project, and then Edit/Paste the media into a newly created project. See if you can share from the new project.
Also, possibly you have some incompatible format in one of your clips, or a corruption in a clip. Check the formats of the clips. iMovie likes Mp4/AAC or mov. You can track down corruption in a clip by expanding your timeline with the slider located at the upper right of the time line, and then slowly scrolling through the project, looking for black frames, white flashes, artifacts, and any other signs of corruption. Cut out any that you find and see if you can share out. Good to work with a duplicate project so that you can preserve your original.
All else failing, here are some general diagnostic procedures that you can go through:
Updating
iMovie
To update/redownload iMovie 10, drag your iMovie 10 app from the Applications folder into the trash, but do not empty the
trash. Sign in to the app store and open your purchases folder by clicking on your name at the bottom of the app store side bar. (NOTE: For El Capitan and older, click on the Purchases tab at the top of the app store screen and then sign in. Click on Purchases again to reveal your purchases.) You will see your purchases displayed, including iMovie 10. Redownload/update iMovie from there. If not successful, drag your present iMovie app out of the trash and return it to the Applications folder.
New
Project
Another thing you can try is to copy your media into a newly created project and see if that cures the problem. To do
that, do an Edit/Select All, Edit/Copy, of the media in your project and then Edit/Paste it into the newly created project.
Safe
Mode
You could also try booting up in Safe Mode (see Finder menu for instructions) and, after everything loads, immediately shut down and reboot in normal mode. Then see if iMovie works properly.
Open iMovie in New User Account
You can try opening iMovie in a new user account that you can create in System Preferences/Users and Groups.
Resetting
the System Management Controller
Try resetting the System Managemen Controller. It is easy to do -- basically restarting while holding down certain keys.
https://support.apple.com/en-us/HT201295
Resetting the NVRAM or PRAM
Try resetting the NVRAM or PRAM, although this issue would be fairly rare. Requires a restart while holding down certain keys.
https://support.apple.com/en-us/HT204063
-- Rich