Crash logs random (and a lot of them) (1.15.2)

The server has worked perfectly for months. Then, at some point, I updated Paper (from 347 to 380) and after a restart:
https://pastebin.com/qWZtb96i (crash logs)

Then I disabled the async chunk load in paper.yml and the server held up for half an hour.
Afterwards, it lagged (as before) and a few seconds later it crashed:
(see the link above)

I restarted it and after a few seconds:
(see the link above)

I tried to use Paper #379 and Paper #349 with a previous backup (edit) but nothing. I use Aikar’s flags with 6GB of RAM. My KVM specs: https://pastebin.com/FrWhmC5A (4 Cores)

Plugin list:

Plugins (55): AntiAFKPlus*, AnvilRestrict, BattlePass, BeastWithdraw, ChatGames*, ChestShop, Citizens, ClearLag, CombatLogX, CommandCooldown*, CoreProtect, CraftingStore, CrazyCrates, DeluxeMentions, DonatorJoinPlus, EpicFarming, EpicFurnaces, Essentials, EssentialsChat, EssentialsSpawn, ezAuctions, FabledSkyBlock, GUIPlus, HolographicDisplays, HolographicExtension, ItemRenamerReloaded, LuckPerms, Matrix, MidniteCore, Multiverse-Core, Multiverse-Inventories, MyCommand, OpenInv, PlaceholderAPI, PluginHiderPlus*, ProtocolLib, PublicCrafters, Rankup, ServerRestorer, ShopGUIPlus, SpigotPing, SuperTrailsPro*, TAB, TelegramBotsAPIPlugin*, TelegramReporter*, UltimateRepairing, UltimateStacker, Vault, ViaBackwards, ViaRewind, ViaVersion, VoidGenerator, VoidSpawn, WorldEdit, WorldGuard

Resolved downgradng UltimateStacker to 1.11.7