I just ran MobHunting-7.5.4 and almost immediately saw an error in the console:
https://pastebin.com/N85FVGDa

My server version: Paper-1.12.2

I can see that you are running a very old server. Is your world very big as well?

My theory is the this error happened because some reward blocks was not loaded. Or may by you have deleted a world?

I cant replicate the problem, but I think I have made a fix for this problem.

please enable debug using "/mh debug"

and then install latest build and check the console output when the reward blocks is saved.

https://fractal.lindegaard.one:8181/job/MobHunting/

I would like to know if this is caused by a deleted world or regenerated world or something? So I can make the best fix.

My world has been re-generated because I am doing a wipe on the server. The radius of the loaded world and WorldBorder is 5000 blocks. I installed the latest build and enabled debug, this is what I saw in the console:
https://pastebin.com/f49SCHxL

Perfect then I know whats wrong. The "bagofgold/rewards.yml" contains all the MobHunting rewards placed all over the world. And because the world has been regenrated the rewards file is pointing to non-exixting blocks.

I would probably just delete the file and restart the server. Doint this will fixed the problem, but I you have placed MobHunting rewards in the new world, they will loose its value.

I will see if I can make a fix for this problem anyway, because you are not the last person who will regenerate his world.

I think i have found a fix. Maybe you can test this for me?

https://fractal.lindegaard.one:8181/job/MobHunting/

The fix should delete all rewards in non-generated chunks.

I already deleted that file and the problem disappeared. But I can replace it in the folder and tell you the result. But I'll do it a little later, as I'm busy right now