Our users are unable to use the Outlook To Do app in the Outlook desktop client.

Just an update from my side...

We have discovered that the IP addresses which the To-Do app connects to vary from user to user.
(As far as I can see, Outlook's To-Do app uses the underlying Webview2 Network Service)

For some users, To-Do connects to "known" O365 IP addresses (52.x.x.x and 104.x.x.x), which are listed by MS:
https://learn.microsoft.com/en-us/microsoft-365/enterprise/urls-and-ip-address-ranges?view=o365-worldwide
For others (including myself), To-Do connects to "unknown" MS IP addresses (20.x.x.x and 13.x.x.x), which are not listed by MS.
(But RIPE identifies them as owned by MS)

For the first group of users, To-Do works in all contexts.
For the latter, To-Do fails with VPN in split tunnel, as the "unknown" addresses are not covered by the tunnel or firewall policy.

However, these addresses change regularly and are within quite large ranges.
So we asked MS for specific address ranges - and classification of these.

Unfortunately, MS has chosen to close the case with the response that they cannot provide us with this information - but that we can test ourselves with whitelisting and tunneling of the addresses we see used.

Best regards
Martin

The hero we needed, it worked on the spot, thank you!

The To Do list feature was working when I uninstalled Office and reinstalled everything yesterday due to some other issues we were having. But then all of a sudden, I was getting this error. These steps did the trick! WooHoo! Thank you.

Reset the Office cache, it should fix the problem:

1. Close Outlook
2. Rename or Remove these two folders: %LOCALAPPDATA%\Microsoft\Office\16.0\wef %userprofile%\AppData\Local\Microsoft\Outlook\HubAppFileCache

Re-open Outlook and click the "To-Do" shortcut, and it should open without issue.

This fixed a user we had. Thank you for this.

This fix was attempted and it did not work for me. On the latest version and build of Version 2307 (Build 16626.20170)

If the problem comes back after doing these steps, check if you have the latest version of Microsoft To-Do on your mobile devices. I upgraded my Android app and the problem on the desktop went away.

YEMV