My client has a button that adds a checklist to a card. Suddenly, these adds are failing.

[Larry Larson]

I have a template with buttons that add checklists to the card. After a year, these butler actions are failing with the error "

Running "Trial" on board "Main Process Board"

Trello error: Request could not be fulfilled. Too many checklists in cards load.

Total operations: 1"

[Michael Pryor]

There is a limit of 15,000 checklists per board.  It sounds like you might have gone over that?

https://developer.atlassian.com/cloud/trello/guides/rest-api/limits/

You could make a new board and move over your active stuff to that board.