Getting 504 Gateway Timeout with "Valid document was not found in the cache and only-if-cached directive was specified" for some tiles

Requesting, e.g., https://a.tile.openstreetmap.org/17/67225/43067.png shows showed:

The following error was encountered while trying to retrieve the URL: http://b.tile.openstreetmap.org/17/67224/43067.png

Valid document was not found in the cache and only-if-cached directive was specified.

You have issued a request with a only-if-cached cache control directive. The document was not found in the cache, or it required revalidation prohibited by the only-if-cached directive.

I really don't think the browser is sending that header. Running the following from a different location (just be sure it's not some network issue at work) yields did yield the very same error:

curl 'https://b.tile.openstreetmap.org/17/67224/43067.png' -H 'Referer: http://127.0.0.1:4200/demo/embed-mjgp2.html' -H 'User-Agent: Mozilla/5.0 (Windows NT 6.1; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/67.0.3396.79 Safari/537.36' --compressed --verbose

...showing no header like that:

> GET /17/67224/43067.png HTTP/2
> Host: b.tile.openstreetmap.org
> Accept: */*
> Accept-Encoding: deflate, gzip
> Referer: http://127.0.0.1:4200/demo/embed-mjgp2.html
> User-Agent: Mozilla/5.0 (Windows NT 6.1; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/67.0.3396.79 Safari/537.36
...
< HTTP/2 504
< server: nginx
< date: Mon, 11 Jun 2018 08:21:34 GMT
< content-type: text/html
< content-length: 1445
< x-squid-error: ERR_ONLY_IF_CACHED_MISS 0
< x-cache: MISS from konqi.openstreetmap.org
< x-cache-lookup: MISS from konqi.openstreetmap.org:3128
< x-cache: MISS from trogdor.openstreetmap.org
< x-cache-lookup: MISS from trogdor.openstreetmap.org:3128
< via: 1.0 konqi.openstreetmap.org:3128 (squid/2.7.STABLE9), 1.1 trogdor.openstreetmap.org (squid/3.5.12)
< strict-transport-security: max-age=31536000; includeSubDomains

So I guess some internal proxy on the OSM server is adding that header?