Managed Challenge 500 Error for POST Requests with sid Query Parameter — Login & Register Pages Neve

Hello Cloudflare Support, Since around a few day ago, POST requests to our phpBB login and registration pages fail with a 500 Internal Server Error generated by Cloudflare’s edge after passing the Managed Challenge. Key facts: This was working previously with no changes to our origin or application. The 500 error is coming directly from Cloudflare’s edge (not our origin). Requests never reach our origin server — no entries in Apache/nginx or PHP logs. The issue occurs only after Managed Challenge on POST requests with form data that include a sid session ID in the query string; requests without sid work fine. Example affected URLs: https://www.wagnardsoft.com/forums/ucp.php?mode=login&sid=... https://www.wagnardsoft.com/forums/ucp.php?mode=register&sid=... Example Ray ID: 96bf31addb65a204-YYZ (register page) Request method: POST, content-type: application/x-www-form-urlencoded. After challenge is solved, Cloudflare fails to replay the POST to the origin and responds with a 500. Note accessing without the &sid=.... work disabling the managed challenge for theses page also work. Steps to Reproduce: Visit https://www.wagnardsoft.com/forums/ucp.php?mode=register&sid=... or ...mode=login&sid=... Pass the Managed Challenge. (usually automatic) Check origin logs — no request is received. Impact: Users cannot log in or register if they are presented with a Managed Challenge and the URL contains a sid parameter, blocking forum access for affected visitors.