A couple weeks ago I bought a game and everything ran fine with the itch.io app. One night I tried opening it and it gave me error. When I look into the details it says something along the lines of "You must purchase this game to run it". I tried uninstalling and then reinstalling the game multiple times. I even went to the initial email I downloaded all of the files from and downloaded them again but it is still not working. I don't know what to do. My device is entirely up to date as well.
Go to the page of the game on itch.io. If you open the game, at the top there should be a banner showing: You own this game.
If you see the banner, then it's likely a glitch with the app.
But most likely you'd have to contact support and give them more details, like the name of the game, a link to the page and your purchase details etc.
Did you try to run the purchased game via the itch app or directly?
What you describe is DRM and that is quite rare on itch. I saw it maybe twice. And to my knowledge there is no support for DRM within the itch app, but maybe I am mistaken. (Meaning it should not matter, from where you run the app)
So basically, complain to the dev, they probably screwed up their DRM scheme, once again proving, that it is legit customers that suffer from that concept.
A likely scenario would be, that the app implements it's drm by phoning home and the server that indie dev put up could not be contacted and your purchase could not be verified.
Of course I can not know what api that is, as you give not exactly much information, but there is html error message 400, bad request. Wich could very well indicate that the game tries to contact a web server and something goes wrong.
As would be consistent with the error the game gives you, it can't confirm your purchase. Itch does not support drm to my knowledge. Not directly at least. But there are some apis apps can use to do that and this involves contacting web servers.
Since I have yet to see a game that does things like that, I can tell you not much beside that something like that exists.
Bottom line is, try to seek out help from the developer. If this is not possible, contact support on itch.
If you go on the game page, in the information section, there should be written a date about last update. This would not be coincidentally be around the time the app stopped working? ;-)
