Since CC is retired this section is slower for people to reply, but I got your message on GameJolt.
Not many people have used GameJolt API in CC I think, especially not myself, but it could be good to try the plugin in a fresh cap to see if it's just not merging with your game or if the plugin truly is broken (are you using Construct Classic R2 from the sourceforge? R1.2 is the outdated version linked from the C2 website).
You will probably need the token to avoid crashing too, that can be found on the top of the GameJolt page here:
Click show, and then odds are the plugin will let you enter it via an event or maybe in the plugin properties, that should be how you authorize the app to work with GameJolt but I have never used the API myself.