If a player has not enabled or disabled InfoHUD, then they are not referenced in the file players.yml and the map ConfigManager.playerMap. Attempting to access these players in ConfigManager.playerMap through the PlayerBedEnterEvent and EntityMountEvent handlers gives an error. Attached are the stack traces.
partial-log.txt

Fixed in 9b7f297 (Version 1.5.2)

Nice catch! I'll try to get it fixed by tomorrow.