Safari 11.1 can't save bookmarks, logins don't work

See if deleting the cache.db associated with Safari helps.

Opne the Finder. From the Finder menu bar click Go > Go to Folder

Then copy / paste the following:

~/Library/Caches/com.apple.Safari

Then click Go then from the Finder window that opens move the Cache.db file to the Trash.

Then quit and relaunch Safari to test.

What happens when you try to log into a web site? New security procedures implemented in Safari 11.1 don't allow auto-fill until you click on the entry box. You will then get an auto-fill option.

Might be a corrupt .plist.

Do a backup, preferably 2 backups on 2 separate drives.

Quit the application.

Go to Finder and select your user/home folder. With that Finder window as the front window, either select Finder/View/Show View options or go command - J. When the View options opens, check ’Show Library Folder’. That should make your user library folder visible in your user/home folder. Select Library. Then go to Safari/Bookmarks.plist. Move the .plist to your desktop.

Restart the computer, open the application, and test. If it works okay, delete the plist from the desktop.

If the application is the same, return the .plist to where you got it from, overwriting the newer one.

Thanks to leonie for some information contained in this.