Just boot into the faulty environment, try the log in a couple of times (perhaps even on a TTY), reboot into recovery, inspect the journald and/or syslog records of the events, if any. Optionally enable debug logging on libpam. The answers should present themselves.
- [arch-general] locked out Fons Adriaensen
- Re: [arch-general] locked out Konnor Klercke
- Re: [arch-general] locked out Nicolás Adamo via arch-general
- Re: [arch-general] locked out Jeanette C. via arch-general
- Re: [arch-general] locked out Jens John
- Re: [arch-general] locked out Fons Adriaensen
- Re: [arch-general] locked out flipee via arch-general
- Re: [arch-general] locked out eye
- Re: [arch-general] locked out Fons Adriaensen