Hate to bring this up here, but I don't have a PMC account. ever since the latest world update that updates the leaderboards, it hasn't been possible to start a new game while one is already running. I believe the same has been happening on the Realms version before the update too.

After some investigation, it seems to be a fixable issue. The issue is within the function kk:items/give, specifically within:

item replace entity @p hotbar.7 with written_book{title:"[Punch] -> Reveal || [Use] -> Game Controls",author:TheAl_T,pages:["{\"color\":\"dark_blue\",\"text\":\"     Game Controls\\n\\n\\n\",\"extra\":[{\"color\":\"dark_green\",\"text\":\"           Easy\\n\\n\",\"clickEvent\":{\"action\":\"run_command\",\"value\":\"/trigger trigger set 1\"}},{\"color\":\"gold\",\"text\":\"          Normal\\n\\n\",\"clickEvent\":{\"action\":\"run_command\",\"value\":\"/trigger trigger set 2\"}},{\"color\":\"dark_red\",\"text\":\"           Hard\\n\\n\",\"clickEvent\":{\"action\":\"run_command\",\"value\":\"/trigger trigger set 3\"}},{\"color\":\"black\",\"text\":\"         Tutorial\\n\\n\",\"clickEvent\":{\"action\":\"run_command\",\"value\":\"/trigger trigger set 4\"}},{\"color\":\"white\",\"text\":\"      Secret Level\",\"clickEvent\":{\"action\":\"run_command\",\"value\":\"/trigger trigger set 5\"}}]}"],HideFlags:24,CanDestroy:["minecraft:white_wool"]}

The title of the book is too long, causing the entire thing to be invalidated. Resolution given to [MC-59153] by Searge says it's intended, so the name just has to be shortened somehow. The exact length allowed seems to be 32 characters, so some compromises have to be made.