All videos show error downloading a higher quality version ?

When you switched to "Download and Keep Originals," it’s possible that the system tried to download large video files that your 256GB storage just can’t accommodate. Even though you have "Optimize Storage" turned back on, the system might be struggling to manage the balance between iCloud and local storage.

First, try manually freeing up some space by deleting unneeded files or apps. After that, go into Settings > Photos, turn off iCloud Photos, and then turn it back on. This should force a resync and might help clear up any syncing or download errors.

If that doesn’t work, you could also try signing out of iCloud and signing back in, which sometimes helps with weird sync issues. Just be sure to back up anything important first, because logging out might remove some of the downloaded files from your device temporarily.