Apple Books won't download any books/files saved in iCloud only on Mac (macOS Big Sur)

Question

I can't download any books/files I saved with iCloud in the Apple Books app on my Mac anymore (it worked before though). Books from the Apple Books store will download fine. I can also download all books saved in iCloud with iOS without problems. There is no error message, instead the Download will apparently start as indicated in the sidebar:

However, the download will never process at all (again, every other download works without issue).

Things I tried that didn't work:

• Rebooted the system
• Logged out and back in with the corresponding Apple account
• Disabled and reenabled syncing for Books in the iCloud settings
• Removed all ~/Library/Application Scripts/com.apple.iBooksX.* folders and rebooted
• Removed ~/Library/Containers/com.apple.BKAgentService/Data/Documents/iBooks folder and rebooted

Answer 1

This seems to be related to the storage optimisation feature for iCloud (maybe it is somehow stuck or blocking these downloads). It can be found under System Preferences -> Apple ID -> Optimise Mac Storage. After turning the future off and on all downloads in Apple Books now work again.