Sometimes, there is no message you can send to a user that will resolve a bad experience. It happens more often than most people would believe. Especially if you are dealing with a buggy product or an organization with a lot of tech debt.
Whether it's a validation error when a person enters their real name, a search that returns no results for a known item, or a crucial document inexplicably missing from a dashboard; dissapointment happens.
The role of the UX Writer in these instances is not to solve the problem—though it would be a good idea to ping the product manager or engineer whose job it is.
The role of the UX Writer here is to to be sensitive to these experiential outliers and do your level best not to shame, iritiate or discourage the user further.
Noticing a theme?
A user who encounters these kinds of errors will immediately be in a vulnerable (and possibly defensive and aggressive) state. They've just had their very existence and importance questioned by a machine; can you blame them?
It's up to you to step into their shoes for a second and write something that's probably not helpful but isn't going to piss them off even further.
For context: according to the 2010 U.S. census, there are 118,285 people in the country with the first name "Oh," and "O" is a valid Korean surname.
The point is—it's probable that there are many people with the (very valid) full name "Oh O" walking around this world.
And, considering fraud detection software sometimes defaults to flagging single character surnames, some names that are quite real could get flagged for being "fake." According to some software, anyone with the last name "O" doesn't exist.
Now would not be the time to deliver the message:
Because you will have, in effect, called a potential user (not to mention a very real person) "invalid."
Whenever you find yourself in this situation, always take an empathetic approach. If you have the character economy—and the product allows for it—link to a "workaround" the user might be able to use. In the case of the login error example using Facebook, Google, Github, Twitter, etc. to create an account would solve this problem very nicely.
Do your best to sound sincere. Guide the user to an alternative path if possible. Never make them feel as if they've reached a dead-end in the experience, even if they "technically" have.