PSA: Starting February 22nd, 2021, anyone with an on-premises mailbox won’t be able to sign in to To Do on any platform.

Had a user at my desk this morning that his newly added tasks disappeared immediately. He had contact with Microsoft support and they said that To Do with on-premises mailboxes would soon be deprecated. Fortunately I only have a couple of users with this combination.

I searched in the 365 Message Center but couldn't find any announcement in the last couple of weeks.

However, you can find it at MS:

Changes to Microsoft To Do access for on-premises mailbox accounts - Office Support

Starting January 22nd, 2021, anyone who uses To Do or the Tasks app in Teams with an on-premises mailbox will only be able to sign in to To Do on the web. You won’t be able to sign in to To Do on Android, iOS, Mac, or Windows. In To Do on the web, you’ll be able to view your lists and tasks, but anything you edit or add won’t be saved and will be lost once you sign out.

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Starting February 22nd, 2021, anyone with an on-premises mailbox won’t be able to sign in to To Do on any platform. Make sure to back up your To Do data before then by printing your lists or saving them as PDFs. Once your organization has moved your mailbox to the cloud, you’ll be able to sign in to To Do and manually reenter the information you saved.

Since I couldn't find it in this subreddit either, thought it would be a good idea to share it here.It's not clear te me yet if it only affects personal tasks and that tasks from a Team (or Planner) will still function.