Page check-out (ctask=check-out) fails with ERR_HTTP2_PROTOCOL_ERROR

Using Concrete / Help
1

Hello Team,

We recently came across an issue with a page on our concrete5 site and we were wondering if there are any ideas as to what could be causing this or how I can go about troubleshoot the cause?

Environment

  • Concrete CMS 8.5.7 (core and installed versions identical)

  • Stack: Caddy (TLS, HTTP/2) → Apache 2.4.65 → PHP 7.3 → Concrete

Steps to reproduce

  1. Log in as a super admin. The CONCRETE5 / CONCRETE5_LOGIN session cookies are present on all subsequent requests, and the edit toolbar renders.

  2. Load a page directly, e.g. https://[REDACTED]/index.php?cID=852 → HTTP 200, renders fully, toolbar visible.

  3. Click the toolbar’s “Edit Mode” link (/index.php?cID=852&ctask=check-out&ccm_token=<fresh-token>).

  4. The navigation hangs ~8–9 s, then Chrome aborts with net::ERR_HTTP2_PROTOCOL_ERROR. Zero response bytes, zero response headers, nothing in the console.

Scope: this only happens on page cID 852 and other pages enter Edit Mode without issue.

Captured on the failing request

  • HTTP/2; session cookies present; fresh ccm_token; correct referer.

  • No response whatsoever — no status line, no headers, no body. The same URL without ctask returns 200 moments earlier.