Hi.
Had the same issue beginning of this year, too.
At first, make sure that your background tasks are using cron and not ajax.
Then i’ve done the following steps and I was able to get the bubbles green on three instances:
php cron.php
php occ dav:sync-system-addressbook
php occ federation:sync-addressbooks
If it does not work at the first try, please try some more often.