Server crashing multiple times

So recently my server has been crashin pretty often(3 times in an hour).

Here is the link to the log - from what I see it's something related to Permissions plugin?

Also I can add that we were out exploring in the nether when the crashes occured.

I'm running version 1.13.2

Plugins used : SilkSpawners, PermissionX, Vault, UltimateTimber, WorldEdit, WorldGuard, ClearLag, Luckperms and LuckpermsGUI