Yeah, it looks like a change made by Trello/Atlassian:
https://developer.atlassian.com/cloud/trello/changelog/#CHANGE-1459
20 February 2024
DEPRECATION NOTICE
Trello API will no longer accept GET request with data in body
Rollout completed March 12th 2024
Trello’s API will no longer permit supplying a body with a GET request. Previous behavior in our API ignored the payloads provided with GET requests but these requests will now be blocked with a 403 response from our content delivery network.
To resolve issues resulting from this change ensure all GET requests to Trello’s API are sent without a payload.
I commented on the issue raised for MMM-Trello, and the author isn’t able to keep up with the development anymore. I’m going to see if I can pick it up, but no promises…