I had the same kind of problem. Every time I wanted to add a new plan I got an error.
At first I thought it was because I changed the template (which activity on which day).
My current plan ends on 22-09-2024 and I had two events on 24-08-2024 and 14-09-2024.
I removed my current plan and also removed the events from the past.
When I retried to add my new plan it went successful!