Minecraft server crash?

Question

Game development

Hello .. I understand that the question is too stupid and asked on the wrong site, but still.
There is a server with a multi-core 1.5-1.8 and a lot of plugins. Hosting flyspring.net
The question itself.
There is a person asking for a certain amount of money in exchange for telling him how to fix a bug that causes him to crash the entire server.
It is not clear how he does it. But there are such things as:
1. A person always comes in from different nicknames.
2. the person always writes /reg 321123 321123
3. after, the person always writes /l 321123
4. after.. the server crashes. sometimes he writes in the chat "Lol"
I attach the last log file of the server crash. crash itself from here - [15:36:08] [AutoSaveWorld CrashRestartThread/ERROR]
I ask to help everyone who understands and who just tells what the problem is. Thank you all in advance!
LOG FILE

Answer 1

1. Try to enter the same yourself to check if this person is doing something else besides entering these two lines
2. If you fail to put the server in this way, then contact those. hosting support and look for information about bugs and vulnerabilities of mods
3. If possible, deploy a server with the same mods on your local computer, try putting
4. If you do, the problem is not in hosting, but in mods. Disable mods one by one and try to repeat the result.
5. If you don't put it on your local computer, it's most likely a hosting problem, write to those. support
Most likely, there is some kind of personal account on the hosting, just in case, change the password there and the security question to recover the password (if any). Also, change your password in the game.