iMovie not loading Photos

iMovie not loading photos on MacBook Pro M2 Pro

I'm working on a MacBook Pro (2023) with Apple M2 Pro, 16 GB; running Squoia 15.6.1. I cannot get iMovie to work. It used to be sluggish to say the least, but now it's completely useless. The Photos library is not loading, so I cannot start a new project. Sometimes the app becomes completely unresponsive, but Activity Monitor shows nothing odd. I've done about anything and everything. At this point I've managed to delete all previous projects, just kept one for testing. Booted, rebooted. Deleted the app. Reinstalled. Now running in safe mode, iMovie froze when clicked on My Media / Photos.

I also have iMovie 10.2.5 running on an old Intel iMac (2,9 GHz Quad-Core Intel Core i5; 8 GB 1600 MHz DDR3)with MacOs 10.15.7. It's snappy, works as expected, except is has no acces to my (iCloud) Photos Library, so I can't just switch.

It's frustrating iMovie is not working on my MBP. Can anybody help, please?