Custom connector to self-hosted WordPress: intermittent "Couldn't connect" / "Couldn't register" errors despite working endpoint

### What happened?

Setting up a custom connector to a self-hosted WordPress site running a custom MCP server (implemented as a WordPress plugin, with OAuth 2.0 + PKCE + dynamic client registration). Repeated connection attempts over several days give inconsistent errors — sometimes "Couldn't connect to the server. Check that the URL points to a valid MCP server," sometimes "Couldn't register with [connector name]'s sign-in service" — each with a different ofid_ reference code, despite no changes to the server between some attempts.

### What did you expect to happen?

The connector should register and complete the OAuth flow consistently. I've independently confirmed the server itself is working correctly: /.well-known/oauth-authorization-server and /.well-known/oauth-protected-resource both serve valid JSON with correct endpoints, and a manual fetch() POST to the /register endpoint, run directly from claude.ai's own browser console, succeeds and returns a valid client_id. This suggests the issue is happening between Claude's connector client and my server, not in the server's own logic.

### Steps to reproduce

1. In Claude, go to "+" → Connectors → Add custom connector
2. Enter server URL: https://www.mygeekmate.com.au/wp-json/mgm-mcp/v1/server
3. Leave OAuth Client ID and OAuth Client Secret fields blank
4. Click "Add" / "Connect"
5. See error (varies between attempts — see Error messages below)

### Area

MCP Connector (adding/managing servers)

### MCP Server (if applicable)

https://www.mygeekmate.com.au/wp-json/mgm-mcp/v1/server

### Error messages or logs

```shell
Error IDs collected across multiple attempts (over several days):
ofid_c0697c5f9e21ac12
ofid_627877aeb59ea2c1
ofid_9210e44402f9dad8
ofid_0e3a01c395ced3ff
ofid_e7b52e478d35b971

Displayed messages (varying between attempts):
"Couldn't connect to the server. Check that the URL points to a valid MCP server."
"Couldn't register with MGM Wordpress Connector's sign-in service."
```

### Additional context

Transport: streamable-http
Client registration type: Dynamic Client Registration (OAuth), client ID/secret left blank in the dialog
Tested in: Claude Desktop (macOS) and claude.ai web
Hosting: SiteGround, behind Cloudflare (proxied)

Verified during troubleshooting:
- SiteGround origin server logs show zero matching requests during failed attempts
- Cloudflare WAF/Security Events show zero blocked requests during failed attempts
- A successful manual POST to /register was logged by SiteGround at the time it was run from the browser directly

This pattern (server works when tested manually, but the actual connector flow fails intermittently with no trace in origin or edge logs) suggests the issue may be occurring before requests reach the public internet path monitored by my hosting/CDN — possibly something in how the connector client itself handles this specific server/response pattern.