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@Nick L. just in case: I managed to confirm that UserStore API calls start to work again if I set one of cookies set by Evernote servers during OAuth procedure: the name of the cookie in my case is web50017PreUserGuid. I guess "web" is prefix here, "50017" is some id specific to each user and/or something else and "PreUserGuid" is a suffix.

Great, thank you very much!

Thank you very much! Contacted @EvernoteHelps on Twitter, hope it would actually help.

Since April 24 2020 something has changed in server side Evernote API handling: calls to UserStore (virtually any calls but in particular checkVersion and getUser) started to return with weird error: HTTP status code is 429 which corresponds to "Too many requests" but the error is "OK". This error has ruined two open source Evernote clients on Linux: NixNote2 and Quentier. Both use unofficial C++ implementation of Evernote API, QEverCloud, but it worked fine for several years and then stopped in a single day on which nothing has changed in the library or in the apps. Furthermore, the error also affects people who use official Evernote SDKs according to this stackoverflow question: https://stackoverflow.com/questions/61404638/why-is-there-a-sudden-increase-in-ttransportexception. There's some investigation on the issue here by NixNote2 maintainer: https://github.com/robert7/nixnote2/issues/151#issuecomment-621384522. In particular, it seems that calls to UserStore started to require some cookie in addition to authentication token and that cookie is set during OAuth procedure. I failed to find anything about this in the documentation. One other thing that I noticed is that www.evernote.com suffers from this problem but sandbox.evernote.com doesn't seem to. I know this forum is user supported but I really hope that someone from Evernote backend team could comment on this issue. Is it a bug? Or is it an undocumented new feature? What should Evernote API users change in their apps to continue working with UserStore?

You might wish to check out another open source alternative that I describe in the neighboring thread. However, note that it's in alpha state now i.e. it's unpolished and too fresh to be considered reliable.