When players join parkour this error appears, and they don't get a scoreboard.
https://pastebin.com/xzwKfQyq

Also, before this bug began, current time wasn't displayed on the scoreboard, even though its enabled in the config.

Scoreboard:
  Enabled: true
  Display:
    CourseName: true
    BestTimeEver: true
    BestTimeEverName: true
    BestTimeByMe: true
    CurrentTime: true
  MainHeading: '&b&l== Parkour =='
  TitleFormat: '&b%TITLE%'
  TextFormat: '&f%TEXT%'

I think this was fixed in commit d257d96 (6.0 branch).

For the scoreboard timer to work, you also have to enable the live timer in config.yml

OnCourse:
   DisplayLiveTime: true

Also the scoreboard manager is enabled when the plugin initialises (when the server starts), so the safest method is:
stop the server;
make the config changes (enable scoreboard and enable live timer as above);
save the config file;
start the server.

Ah so that needs to be enabled too.
Let me build the newest version of the plugin and see if that fixes the problems.

Yeah seems to be working fine now.