How do I fix repeated 'Error 400' in iCalendar for a single contact after adding their birthday?

I have recently added a birthday to one of my old contacts in my gmail account. Since then I receive repeated Error "400" in my iCalendar for that single person and the gmail account this contact resides in.

This is a similar issue that others reported, some of the previous suggestions were over my head although I have reviewed some of the information shared.

The previous suggestions from 2023 suggested CalDav accounts which both of my gmail calendars that I am syncing with my iCalendar are set as.

System:

MacBook Pro M1 2020

macOS Sequoia 15.3.2

[Re-Titled by Moderator]