That hostname points to Fastly IP addresses which return a 302 redirect to https://error.ghost.org/. Not related to Cloudflare you likely need to take it up with the hosting provider (ghost.org I assume).
Not sure where you’re seeing a 526 error.
❯ curl -Ikv https://polymath.net
* Trying 151.101.195.7:443...
* Connected to polymath.net (151.101.195.7) port 443 (#0)
* ALPN: offers h2
* ALPN: offers http/1.1
* (304) (OUT), TLS handshake, Client hello (1):
* (304) (IN), TLS handshake, Server hello (2):
* (304) (IN), TLS handshake, Unknown (8):
* (304) (IN), TLS handshake, Certificate (11):
* (304) (IN), TLS handshake, CERT verify (15):
* (304) (IN), TLS handshake, Finished (20):
* (304) (OUT), TLS handshake, Finished (20):
* SSL connection using TLSv1.3 / AEAD-CHACHA20-POLY1305-SHA256
* ALPN: server accepted h2
* Server certificate:
* subject: CN=fallback.tls.fastly.net
* start date: May 28 23:05:13 2023 GMT
* expire date: Jun 27 23:05:12 2023 GMT
* issuer: C=US; O=Certainly; CN=Certainly Intermediate R1
* SSL certificate verify ok.
* Using HTTP2, server supports multiplexing
* Copying HTTP/2 data in stream buffer to connection buffer after upgrade: len=0
* h2h3 [:method: HEAD]
* h2h3 [:path: /]
* h2h3 [:scheme: https]
* h2h3 [:authority: polymath.net]
* h2h3 [user-agent: curl/7.86.0]
* h2h3 [accept: */*]
* Using Stream ID: 1 (easy handle 0x128014800)
> HEAD / HTTP/2
> Host: polymath.net
> user-agent: curl/7.86.0
> accept: */*
>
< HTTP/2 302
HTTP/2 302
< server: openresty
server: openresty
< content-type: text/html
content-type: text/html
< location: https://error.ghost.org/
location: https://error.ghost.org/
< ghost-cache: HIT
ghost-cache: HIT
< cache-control: no-cache, private, no-store, must-revalidate, max-stale=0, post-check=0, pre-check=0
cache-control: no-cache, private, no-store, must-revalidate, max-stale=0, post-check=0, pre-check=0
< ghost-age: 410
ghost-age: 410
< x-request-id: 19fb0fc2-7bca-486c-9725-7973541ac94d
x-request-id: 19fb0fc2-7bca-486c-9725-7973541ac94d
< accept-ranges: bytes
accept-ranges: bytes
< via: 1.1 varnish, 1.1 varnish
via: 1.1 varnish, 1.1 varnish
< date: Mon, 05 Jun 2023 06:05:47 GMT
date: Mon, 05 Jun 2023 06:05:47 GMT
< x-served-by: cache-ams12747-AMS, cache-ewr18143-EWR
x-served-by: cache-ams12747-AMS, cache-ewr18143-EWR
< x-cache: MISS, MISS
x-cache: MISS, MISS
< x-cache-hits: 0, 0
x-cache-hits: 0, 0
< x-timer: S1685945147.104438,VS0,VE93
x-timer: S1685945147.104438,VS0,VE93
< vary: Cookie
vary: Cookie
< ghost-fastly: true
ghost-fastly: true
< alt-svc: clear
alt-svc: clear
< content-length: 142
content-length: 142
)