We are having the same issue with an app for which Ads API has been recently enabled. Not all the users are having the issue, e.g. my personal username @frangarcia is able to authenticate properly (be aware that I had already authenticated my account against this app previously) but a brand new username @frangarciawi just created this morning is not able to authenticate and I get the error
The application cannot authenticate more users at this time
Definitely, is not only affecting brand new accounts as I have some other twitter users complaining about not being able to authenticate and I have just checked they are old twitter accounts.
I have just followed you in case you need more details via DM.