Having same issue, everything seemed to be working fine and then 2-3 days ago seemed to error.
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So I reached out to Google and have been provided with the following steps (please note we are using G-Suite Enterprise so this fix may not apply to everyone..)
In Admin Console, go to Security > Settings > API permissions (at the bottom) > Calendar (the apps installed),Once you've clicked on the Apps, you'll see 2 lists: Installed, and Trusted.
Search for the app in the Installed list, click on the setting on the right side, and then Trusted.
Move the App to the Trusted list to see if this solves it.
I confirmed today that this did solve the issue for my users
Thanks for sharing this, Gareth