I know I am not a staff member but I think I have the solution to this.
Since I have my own Minecraft server, I know how certain plugins work. Because of the message appearing on the login, I know what plugin you use for Authentication. On that plugin, there is the option not to allow a new player with the same name as someone else but with different capitalization, and since the usernames here are exactly the same but with different capitalization, the plugin won?t allow you to join with a different capitalization username. What I believe could fix this is resetting the login account information through the admin commands, and that will delete the player from the plugin's database, therefore allowing a you to join with the other username. The command is "/lac rmpass goshenkata" I think, a staff member has to do the command, it is not accessible through a regular member.