Specific user can log into an externally-authenticated FMP-hosted database but can in others

[21st Century Man]

(using FM Server v21.1.1.40, FMP client v21.1.1.41)

This is a head scratcher: My boss can log into all but one of our databases using External Server and FileMaker File (the two types in the Authentication window), but he can't log in via External Server in a relatively newly created FMP file; he can log in using FileMaker File. His credentials haven't changed, and he has no saved passwords in Keychain (on his MacBook). The error message he gets is "The provided account name and password cannot be used to access this file."

The file is suspect, but its permissions setup is the same as in all the other databases so I'm at a loss where to go with this. Any ideas on what I can do/check?