Excellent, glad to hear you were able to get it working eventually, and thanks for the info, helps a lot as I keep an eye on this. The region issue is certainly a good theory; while I get purchases from all over, the majority are still in the US, as am I, so that might also go to explain why I haven't had more complaints, and why I'm not able to reproduce the issue.
Allow me to further speculate. This gives some interesting additional info: apparently Itch just uses google cloud storage, which is blocked in a variety of settings. So it's not even an issue of Itch having issues with their servers: Itch DOESN'T HAVE servers. It just rents space from Alphabet. The UK in particular is known to massively filter and censor their internet on a national scale, for a variety of Orwellian reasons, and it wouldn't be surprising if file lockers like google cloud are on one or more shitlists with the UK's censorship orgs, probably more for enabling filesharing than anything else. Alternatively, they may not be trying to fully block it, but their filters may be causing some kind of unexpected glitchiness: needless complexity in software always increases bugs.
Disregard, I'm an idiot, that was an ancient post. Itch DOES have its own CDN, they just migrated recently, and this caused issues. As of now, it SHOULD be mostly resolved according to them.