This is a recurring bug with users not being able to log in, due to inconsistent state between CentralAuth and local wikis (specifically the local user table row missing in the latter).
Problem description: The user gets an exception whenever they try to login or get an exception when opening the CentralAuth page (of the user): https://en.wikipedia.org/wiki/Special:CentralAuth/USERNAME
For user which this affects
A possible fix: Enter your username at testwiki:Special:CentralAuth. Look for text of form Could not find local user data for username@X, where username is your name and X is the name of a wiki. Try to log in at that wiki. For example, if X is bewiki then enter be: in the search box to get to the be: wiki, or look for the wiki at Special:SiteMatrix. The language may be foreign but the login link should be at the top right or top left, or you can enter Special:UserLogin in the search box.
This description was edited to reflect the actual issue, and not only about the first affected users situation. ~@Josve05a