If you encountered it without a VPN, it would appear to not be based purely on VPN usage - which at the very least is good news for the privacy oriented.
It's not a bug. It's them running A/B tests to completely block mobile users that aren't on the app. They've been implementing different versions of this for over a year now. They always lie about what it was. Actually they pretty much habitually lie about all of their tests and changes.
I got this bug on desktop, running chrome, with no VPN.
This was, as most things like this are, just a pure dumb fuck-up by some guy putting things on prod without properly testing and staging. No need to put on any more tin foil hats than we already have, the incompetence is plenty reason enough to point and laugh.
Reddit has so many users now, that they can filter out everyone who is in the very least problematic or not fully profitable, and still sell ads to millions.
The only time they'll do something that reduces profits is when they're confident it will mean more profits in the near future (and they can't figure out a way to have both).
That's why they were happy to platform mask off neo-nazis, the dangerously stupid and communities dedicated to getting as close to child pornography as possible without technically breaking any laws and why they waited until the last possible moment to pull the plug on them.
It looks like most of you had difficulty reaching the site for about 5 minutes, but those issues should have subsided.
During that time, you may have been shown an incorrect error message that read:
Whoa there, pardner! reddit's awesome and all, but you may have a bit of a problem.
Make sure your User-Agent is not empty, is something unique and descriptive and try again. if you're supplying an alternate User-Agent string, try changing back to default as that can sometimes result in a block.
To share some additional context on what happened - we pushed a bad code change in our tooling that resulted in a significant amount of users getting blocked without doing anything wrong. So if you happened to see that error message within the last hour, don't fret! We've reverted the code change that caused this error and things should be back to normal very soon if they aren't already.
Additional edit: Reddit has written an official post stating it was an issue after production code deployment and was reverted
In their mind the issue was not the blocking of users, but that the users have started complaining. I bet they just reverted it so that they can bring it back slowly, after just checking whether it is effective at all
Seriously doubt this is an API issue. The error itself would make it seem like it's a problem with your browser, but unless you have some overzealous privacy extensions installed that are hiding your user agent, it's more likely to be a temporary problem on their side.