Steps to recreate:

1. Create a new Spigot plugin (API version 1.20)
2. Create a new command in said plugin.
3. In the command, ban the player that ran the command using Player#ban.
   I have tested with both Duration and Date variants of the method.
4. After the ban expires, join the server.
5. See that an internal server error has occurred and that the player did not successfully join the server.
6. See that on the next join attempt, the player can join as normal.

Stack trace attached.