iMovie won't shut down "iMovie must complete other tasks before performing this operation" Over 24 hrs, stuck.

Done editing one of multiple projects, but iMovie won't shut down.

Pop up message:

" iMovie must complete other tasks before performing this operation. Try again once these tasks have been completed. You can view their progress by clicking the Activity Indicator in the toolbar."

Appears to be stuck. Over 24 hours elapsed, iMovie still won't shut down. Using"Quit iMovie" returns this message.

Note:

• My projects are using Media from iCloud Photo library.
• There's no Activity Indicator appearing on the toolbar.
• This has happened before, on a previous project. There was an Activity Indicator icon on the toolbar, which showed progress on a "circle-clock" progress indicator, returned a message something like "iMovie is processing, wait a few moments." Leaving the iMac for a few hours seemed to allow this to complete. I could shut down later, using Quit iMovie.

iMac 24 2023

Memory 16 GB

Apple SSD (500MB used of 2TB)

Sequoia 15.3.1

5G Wireless (test during this problem: >500 mps down, >17 mps upload)