I have a solution, but it’s more of a workaround, though it stopped the problem.
What I did was to add manually the fields via phpMyAdmin. These are the values I’ve inputted for the new columns:
Name – Type – ?NULL? – Default
team_link – tinyint(1) – Yes – NULL
default_time – datetime – Yes – NULL
player_mod – tinyint(1) – Yes – NULL
prediction_mod – tinyint(1) – Yes – NULL