The issue with G Suite user sync is almost always related to synchronisation admin to loosing permissions in G Suite. To find out what is causing the issue, we recommend using the G Suite Admin Audit panel to check what was changed with you sync admin permissions and revert the changes.
To access the G Suite Admin Audit, you need to have G Suite admin permissions and go to this page: https://admin.google.com/AdminHome?pli=1&fral=1#Reports:subtab=admin-audit
In there you can filter the changes with the sync admin email address and then revert the changes from the G Suite admin panel.
After the sync has been restored, please contact our support and lets work together if you need to modify your sync admin or change the sync admin permissions in G Suite.
You can find more about the recommended permissions in our article about Installing Happeo.