I might have discovered the cause.

The FAQs on catbox.moe have this to say:

Connectivity Issues Why can’t I access Catbox/access files on Catbox (files.catbox.moe)? Good question! It turns out certain governments in the world hate freedom of information, don’t have net neutrality, don’t see online service providers as legally exempt entities, or a combination of the three!

You will have issues accessing Catbox/files on Catbox if you are connecting from the following countries or providers:

Australia

Why? - The Australian government had a heart attack after the Christchurch shooting, and decided that Catbox was a terrorist propaganda spreading website, and has instructed multiple Australian ISPs to implement DNS blocks of Catbox’s domains. How to resolve? - Changing your DNS servers to a public server (i.e. 8.8.8.8, 76.76.2.0, or 208.67.222.222) or utilizing DoH should mitigate this problem.

Ireland/UK

Why? - Unfortunately the love of footy has overtaken the sensibilites of the British and Irish governments, and they have decided to implement an IP level block on Catbox’s primary IP address, due to clips of games being uploaded to Catbox… How to resolve? - You will need to utilize a VPN in order to evade this traffic hijack.

Iran

Why? - Unknown, but I’m sure you can probably make some pretty accurate guesses. How to resolve? - You will need to use a VPN to bypass the IP-level block.

Afghanistan

Why? - Unknown, but I’m sure you can probably make some pretty accurate guesses. How to resolve? - You will need to use a VPN to bypass the IP-level block.

Comcast/Comcast Business

Why? - Comcast Business’ “SecurityEdge” hijacking attack intecepts traffic going towards Catbox. Residential customers are blocked by “xFi Advanced Security” I have been unable to determine if this is a DNS level block, or an IP level block. How to resolve? - Comcast Residential: Disable the “xFi Advanced Security” hijack on your router. Comcast Business: Disable the “SecurityEdge” hijack on your account. See here.

Quad9

Why? - Quad9 actually pulls an aggregate list of URL blacklists and uses them for their filtered DNS service. Catbox is frequently added to no-name blacklists for various reasons. How to resolve? - Use Quad9’s alternative DNS server 9.9.9.10, or switch to another public DNS server.