It seems that in 2021 Google will enforce the use of Oauth and block access to "Less secure apps"(LSA)
They sent an email to all admins to take actions.
This is part of their announcement,
Access to LSAs will be turned off in two stages:
June 15, 2020 - Users who try to connect to an LSA for the first time will no longer be able to do so. This includes third-party apps that allow password-only access to Google calendars, contacts, and email via protocols such as CalDAV, CardDAV and IMAP. Users who have connected to LSAs prior to this date will be able to continue using them until usage of all LSAs is turned off.
February 15, 2021 - Access to LSAs will be turned off for all G Suite accounts.
Let's hope Zentyal can solve this on time!