HOST131: Response headers rejected
What this means
A proxy in front of the store rejects the server's responses for having too many headers.
A cache or proxy layer (commonly Varnish) enforces a limit on response header count or size. The store's lightweight endpoints fit, but full API responses exceed the limit and come back as 503s from the proxy, not from WordPress. The limit must be raised on that layer.
Your data
No order or product data is affected.
Troubleshoot
- On Varnish, raise http_max_hdr (header count) and http_resp_hdr_len / http_resp_size (sizes).
- Ask the host which proxy layer returns the 503 — the WordPress error log will show nothing because the request never fails there.
- A quick check: the store's /wcpos/v2/ping answers fine while data endpoints 503.