This is happening in two sites I've restored using BackupBuddy*. One is single, the other Multisite.
.* this may render the question as "too localized", but maybe it could happen with any other backup/restore tool
Whenever upgrading (single or bulk), the message appears: "The update process is starting", the spin keeps spinning, but doesn't goes past this point.
If bulk upgrading, only the first upgrade is shown as in progress.
I give it a few minutes to be sure. All updates were successful and the site is not in maintenance mode.
WP_DEBUG doesn't report nothing. The server error_log don't have any info. The browser console don't throw any errors.
I tried to delete the
update_plugins transients, but now the update gets stuck in the message: "Downloading update from..."
It completes anyway.
Looks like all of this is handled inside the file
/wp-admin/includes/class-wp-upgrader.php, but I can't find any hint inside of it.