I don’t recall seeing this particular error. You are most likely to find the offending Calendar account listed here:
Settings > Passwords & Accounts
If you are unable to locate an account that corresponds with the problem, perhaps a forced-restart of the iPad will help - as this often resolves isolated issues:
- On an iPad with a Home button: Press and hold the top button and the Home button at the same time. When the Apple logo appears, release both buttons.
- On an iPad with Face ID: Press and quickly release the volume up button, press and quickly release the volume down button, then press and hold the top button. When the Apple logo appears, release the button.
If still unresolved, you may be best advised to seek assistance directly from Apple Support.
The most expedient route to reaching the Support Team is to either use the Contact Support link at top-right of this page - or use the Apple Support App (which may already be installed on your iPad) to initiate a support call:
https://apps.apple.com/gb/app/apple-support/id1130498044
I hope this information and guidance proves to be helpful in resolving the problem.