Broken here, too, but in a different way. Time machine seems to complete the backup, but it's not really complete. No errors, but the date of last backup doesn't change and when it tries next time it attempts to back up the same stuff; so it's like it's not finishing properly. It transitions from Prepariing Backup to actually updating the progress bar as it copies, and it looks like it completes, but no luck.
UPDATE:
Found a workaround. Shut off automatic backups. It was a large incremental backup due to the OS upgrade, so it was taking a long time (over an hour). I suspected that the automatic hourly backup triggering during the process may interfere with normal backup. That seemed to solve the problem and the manual backup completed normally.