Hi NicoARG1974,
Thanks so much for using Apple Support Communities. It's great to have you here.
What device are you experiencing this issue with and what version of software is installed on that device?
Also, if you use the steps in the following article to turn any calendar accounts off and on again, do you continue to experience this issue?
Add or delete calendar accounts on Mac
Set up multiple calendars on iPhone
Once you've completed that step, restart your device and test to see if you if that error message continues to pop-up.
Restart your iPhone
How to turn your Mac on or off
Cheers!