Batch API error: ResponseText: {"errors":[{"message":"Not acceptable format: json"}]} When running a batch process on Drupal 8, the following error is returned:

ResponseText: {"errors":[{"message":"Not acceptable format: json"}]}


The HTTP response is HTTP POST / 406

I can see the route is defined ok for the batch callback with the JSON support there:

select * from router where name like '%system.batch_page%' \G;
*************************** 1. row ***************************
           name: system.batch_page.html
           path: /batch
pattern_outline: /batch
            fit: 1
          route: C:31:"Symfony\Component\Routing\Route":970:{a:9:{s:4:"path";s:6:"/batch";s:4:"host";s:0:"";s:8:"defaults";a:2:{s:11:"_controller";s:52:"\Drupal\system\Controller\BatchController::batchPage";s:15:"_title_callback";s:57:"\Drupal\system\Controller\BatchController::batchPageTitle";}s:12:"requirements";a:2:{s:7:"_access";s:4:"TRUE";s:7:"_format";s:4:"html";}s:7:"options";a:4:{s:14:"compiler_class";s:34:"\Drupal\Core\Routing\RouteCompiler";s:12:"_admin_route";b:1;s:12:"_no_big_pipe";b:1;s:14:"_access_checks";a:1:{i:0;s:20:"access_check.default";}}s:7:"schemes";a:0:{}s:7:"methods";a:2:{i:0;s:3:"GET";i:1;s:4:"POST";}s:9:"condition";s:0:"";s:8:"compiled";C:33:"Drupal\Core\Routing\CompiledRoute":312:{a:11:{s:4:"vars";a:0:{}s:11:"path_prefix";s:0:"";s:10:"path_regex";s:12:"#^/batch$#sD";s:11:"path_tokens";a:1:{i:0;a:2:{i:0;s:4:"text";i:1;s:6:"/batch";}}s:9:"path_vars";a:0:{}s:10:"host_regex";N;s:11:"host_tokens";a:0:{}s:9:"host_vars";a:0:{}s:3:"fit";i:1;s:14:"patternOutline";s:6:"/batch";s:8:"numParts";i:1;}}}}
   number_parts: 1
*************************** 2. row ***************************
           name: system.batch_page.json
           path: /batch
pattern_outline: /batch
            fit: 1
          route: C:31:"Symfony\Component\Routing\Route":858:{a:9:{s:4:"path";s:6:"/batch";s:4:"host";s:0:"";s:8:"defaults";a:1:{s:11:"_controller";s:52:"\Drupal\system\Controller\BatchController::batchPage";}s:12:"requirements";a:2:{s:7:"_access";s:4:"TRUE";s:7:"_format";s:4:"json";}s:7:"options";a:3:{s:14:"compiler_class";s:34:"\Drupal\Core\Routing\RouteCompiler";s:12:"_admin_route";b:1;s:14:"_access_checks";a:1:{i:0;s:20:"access_check.default";}}s:7:"schemes";a:0:{}s:7:"methods";a:2:{i:0;s:3:"GET";i:1;s:4:"POST";}s:9:"condition";s:0:"";s:8:"compiled";C:33:"Drupal\Core\Routing\CompiledRoute":312:{a:11:{s:4:"vars";a:0:{}s:11:"path_prefix";s:0:"";s:10:"path_regex";s:12:"#^/batch$#sD";s:11:"path_tokens";a:1:{i:0;a:2:{i:0;s:4:"text";i:1;s:6:"/batch";}}s:9:"path_vars";a:0:{}s:10:"host_regex";N;s:11:"host_tokens";a:0:{}s:9:"host_vars";a:0:{}s:3:"fit";i:1;s:14:"patternOutline";s:6:"/batch";s:8:"numParts";i:1;}}}}
   number_parts: 1
2 rows in set (0.00 sec)


Just wondering if anyone else has encountered this error for the Batch API processing and may have some suggestions on it.
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