Server Crash

So, let me tell you the story.

I was inside my house and I decided to go outside, but when I went outside there was plenty of mobs just chilling in the entrance, I died to a skeleton and I think it was affected with the "champions" mod cause he had a boss health bar. So when I died, I quickly hit respawn but my Minecraft just stopped responding and it kicked me from the server, the server crashes afterwards and I just left the server alone for around 30 minutes. 20 more minutes later, it was 50 minutes total in after the server crashed and kicked me, my friend told me he was gonna start the server and play, but when he joined the server crashed. And 10 minutes later, I hopped on Minecraft and tried if mine works, yep you guessed it, same outcome the server just crashes when a player joins.

Here's the crash report: https://mclo.gs/fGU5cY4.

Right, so moving on I asked one of my friend with this problem, and he suggested to remove every config file and let it regenerate by running the game again, but, we didn't know which config file we needed to delete tho, so I tried something and used an automatic backup from yesterday, the server works! But this isn't the end, why am I creating this forum if the problem is fixed? So after messing around, everything works fine when I used the backup from yesterday, and just in case, I saved a backup for the one that crashes the server when a player joins, so I used the backup that crashes the server it still doesn't work, yeah what did I even expect. So then I decided that I'm going to stay and use the backup from yesterday, not much progress was lost it was just ore stuff. I decided to close Minecraft for a while and I decided to hop back on again, yep tried the crashing backup, and the working same outcomes. So I was still playing in my working backup, right? After some time, I died to a zombie (Thanks ImprovedMobs Mod!) and when I hit that respawn button, uh oh! It's the same thing over again, the server crashed and it kicked me!

I had enough of this crashing, so I'm right here typing this forum with my best manners and formal grammar. I have not yet started the server after getting killed by the zombie, but I bet that the server will still crash even if I try. And if it were to work, I still want to fix the issue with the server crashing when getting killed by an entity.

(There's also another problem with my friend trying to join the server, but fails, this is after I used the automatic backup from yesterday. He said along the lines of "Out of memory", "It's always stuck at the loading terrain", "It said took too long to log in" "Saving world" "Minecraft was closed due too many memory in java".)

So a little of update, I had a gut feeling that it would work when I try while writing this forum, and it actually worked, my server didn't crash when I joined. But I still want an answer to this tho, so that it wont happen again in the future.

Is there any possible way to fix this problem, if anyone know something, please tell me and let me know. Sorry if this is WAY too long.
Cheers!

Will try tomorrow, I'll let you know if my friend's issue is fixed. But do you have any idea about the issue about the server crashing when a player gets killed?

Alright, I just wanna ask, how do you I reduce the server load? Like examples of things I can do to reduce it.

Forgot about this forum. Everything's normal now, server doesn't crash if a player dies, and the crashing issue when my friend joins never happens too. A couple of new problems has arise, but it can be fixed pretty easily.

Would appreciate it if someone close the thread, Cheers!