Substack is currently blocking Overcast.
It looks accidental, via Cloudflare misconfiguration. I'm trying to get in touch with them.
I cannot stress enough to podcast hosting companies: DO NOT HOST PODCAST FEEDS BEHIND CLOUDFLARE!
If you do, you MUST create a Cloudflare rule that changes the "Security Level" to “Essentially Off" for all feed URLs. Otherwise, if you host many feeds (especially per-member feeds), most podcast apps will be throttled or blocked from accessing them.