As y'all can imagine, I am constantly beating on my Safari / iCloud bookmarks, often poking at them with outlandish reverse-engineering experiments to see what happens.  As a result, the Safari / iCloud bookmarks are often broken.  The last week or so I have sometimes been getting intermittent You can't change bookmarks now dialogs when attempting to change bookmarks.  Although this problem has never been reported by users of our apps, I'm posting what I learned today, because I have seen this reported in other forums, with no good answers.

To fix You can't change bookmarks now, you should, first of all, wait a few minutes as the message advises and try again.  Then if you still have the issue, you can either log out and then back in to your Mac account, or try this slightly less extreme fix:

• Quit Safari.
• Activate Finder.
• Click in the menu: Go > Utilities.
• In the window that opens, find and launch Activity Monitor.
• Into the search field of the window that opens, enter SafariBookmarks (all one word).  The list should be shortened to reveal SafariBookmarksSyncAgent.
• Click to select SafariBookmarksSyncAgent.
• Click the "X" button in the toolbar.
• In the sheet which appears, click "Quit".

That fixed it for me, twice.  I checked to see if quitting either Safari or SafariBookmarksSyncAgent alone would fix the problem, but it did not.  It seems you must quit both of them, as described above.

UPDATE 2021-FEB-03.

Later, around February 2020, I had the problem again, but this time, the above fix did not work.  I could edit bookmarks after quitting SafariBookmarksSyncAgent, but upon logging back out and back in, SafariBookmarksSyncAgent would launch again, and then I could not edit bookmarks any more.  And my bookmarks never synced to my other device, an iPad Mini Retina 2 which is running iOS 12 which is the latest system it will support.  I called Apple Support a half dozen or so times.  I explained to them what I thought the problem was, which is that my Safari bookmarks *in* iCloud contained corrupt data, and whenever I switched on Safari syncing and iCloud attempted to push this corrupt data down to my Mac, some error would occur and it would get stuck in an infinite loop or something.  I told them: You need to clear out and “reset” my Safari bookmarks in iCloud.  But the agents all told me they had no way to do this.

I think I was given a case number, called back every few weeks for a few months and was repeatedly told that there was no progress.  Finally, I was told that this was a known bug, implying that it was affecting other users as well, and it was in the queue for “engineering” to fix it, but there was no expected completion date.  So I gave up.

Since then I have replaced my old MacBook Air with a new Apple Silicon “M1” MacBook Air running macOS 11, and added a third device, a 7th Gen iPod Touch running iOS 14, to my iCloud account.  Today, I tried the above fix again and found that, to my surpise, after logging out and back in on my Mac, I am, for the first time in about a year, able to edit bookmarks while SafariBookmarksSyncAgent is running, and my Safari bookmarks are actually syncing between my new MacBook Air and new iPod Touch.

But they are not syncing to my old iPad even after restarting that device and switching its Configuration > Apple ID > iCloud > Safari on and off, and relaunching iOS Safari many times.  When switching it on, even if I respond to delete all of the current bookmarks on the iPad, nothing is deleted.  The bookmarks in my old iPad are off in their own island.

I recall that, a couple years ago, Apple made some advancements in their iCloud system, replacing old thing with their new thing called “CloudKit”, but this was only supported by devices running newer operating systems, not my old iPad.  Somewhere I’ve seen dialogs asking if I want to “migrate” my data to CloudKit, with a warning that old devices may no longer work, to which I responded No.  So maybe Apple forcibly upgraded my Safari bookmarks to CloudKit, which fixed the issue on my Mac, but left my old iPad behind.  I don’t know.  This is all wild speculation.  We would appreciate anyone posting their own experiences with this issue.

UPDATE 2021-Feb-12

One of our users reports that, also for them the procedure given in this post did not fix the probme.  But an IT pro … from a local Mac store was able to fix it.  We do not know what was done.

UPDATE 2021-Mar-06

Well, the problem is back.  Judging from the bookmarks I see in Safari on my Mac vs. the bookmarks in Safari on my iPod Touch, iCloud syncing apparently stopped working again sometime between 2021 February 7 and this morning, 2021 March 6.  When I visit https://iCloud.com, click "Account Setting", scroll down to "Advanced" and click "Restore Bookmarks", after spinning for many minutes, the Choose the bookmarks you would like to restore list appears but it is empty.  So iCloud bookmarks on the web are on the fritz too.