Batch API error: ResponseText: {"errors":[{"message":"Not acceptable format: json"}]}
In our case, we had to turn off Cloudflare's Query String Sort option. See here --> https://support.acquia.com/hc/en-us/articles/360022037774-Resolving-Not-acceptable-format-json-error-HTTP-code-406-in-batch-operations

Here is the drupal issue too --> https://www.drupal.org/project/drupal/issues/2879023