Summary:
This problem was related to encoded versions of the title parameter in the ATS DC routing logic. Some clients URL-encode : and / characters. Those requests weren't forced to the primary DC, which caused race conditions in reading & writing the token cache (main stash). It's unclear why this only started happening after the DC switchover.
The patch added variants, such as Special%3AOAuth%2Ftoken (for Special:OAuth/token). The patch also added a missing /authenticate OAuth 1.0 endpoint
Original task description:
Several external tools appear to be having issues with logging in with OAuth. All of the following will eventually work, but it might require N tries before you're actually logged in:
I only have access to CopyPatrol and was able to deduce it's OAuth that's failing. The JSON response we get back is:
{ "error": "mwoauthdatastore-bad-token", "message": "No token was found matching your request."} }
Make enough attempts and you will get back a token. This matches the reports for The Wikipedia Library (T332349#8707537) and other OAuth clients (T332429). These seem very similar and hint at a broader issue.
Interestingly, other tools like Global Search and XTools seem to have no issue with logging in, and they're using the same package as CopyPatrol (mediawiki/oauthclient). All are using OAuth 1.0a as far as I can tell.