My server sometimes crashes and I dont know why.

It seemed Oxide was reloading for some reason, then the crash happened I think on InstantCraft loading skins? But I dont understand why it needs to load skins?

13:42 [Info] [Death Notes] [DOGI] P A N I C W A V E shot a Boar using their LR-300 Assault Rifle over a distance of 1.7 meters.
13:42 [Info] [Rewards] [DOGI] P A N I C W A V E got 375 for killing a boar
13:42 [Info] [PrivateMessage] [PM][rags] das.Kloetenpony->[rags] carpe diem: ok, afk for tea too :)
13:42 [Info] [Better Chat] [NEWB] markieboy123: hoe moet ik mijn clan leaven
13:42 [Info] [Death Notes] 그는 인사하지 않았다Xia shot [BATZ] Maxibaobab using their LR-300 Assault Rifle over a distance of 4.9 meters.
13:42 [Info] [Rewards] 그는 인사하지 않았다Xia got 1500 for killing [BATZ] Maxibaobab
13:43 [Info] [Better Chat] [Admin] [God] Professor Goldsteinberg: use /clan help
13:43 [Info] [Death Notes] Memory dust shot KaSp3r_8 using their LR-300 Assault Rifle over a distance of 1.7 meters.
13:43 [Warning] Calling 'OnPlayerInit' on 'RustIO v2.15.0' took 173ms
13:43 [Info] [Death Notes] Khan had enough of life.
13:43 [Info] [Better Chat] Sebastian118811: hello
13:43 [Info] [Better Chat] [Admin] [God] Professor Goldsteinberg: Hello
13:43 [Info] [Death Notes] [BATZ] DaDi shot aanillo30 hellcase.com using their LR-300 Assault Rifle over a distance of 0.9 meters.
13:43 [Info] [Rewards] [BATZ] DaDi got 1500 for killing aanillo30 hellcase.com
13:43 [Info] Loading Oxide Core v2.0.3867...
13:43 [Info] Loading extensions...
13:43 [Info] Latest compiler MD5: 5fbca0bf39b1c09ddf2fb3686b869242
13:43 [Info] Local compiler MD5: 5fbca0bf39b1c09ddf2fb3686b869242
13:43 [Info] Loaded extension CSharp v2.0.3913 by Oxide and Contributors
13:43 [Info] Loaded extension Discord v1.0.0 by PsychoTea & DylanSMR
13:43 [Info] Loaded extension Rust:IO for Oxide v2.15.0 by playrust.io / dcode
13:43 [Info] Loaded extension MySql v2.0.3755 by Oxide and Contributors
13:43 [Info] Loaded extension Rust v2.0.4023 by Oxide and Contributors
13:43 [Info] Loaded extension SQLite v2.0.3754 by Oxide and Contributors
13:43 [Info] Loaded extension Unity v2.0.3756 by Oxide and Contributors
13:43 [Info] Using Covalence provider for game 'Rust'
13:43 [Info] Loading plugins...
13:43 [Info] Loaded plugin Rust v2.0.4023 by Oxide and Contributors
13:43 [Info] Loaded plugin Unity v2.0.3756 by Oxide and Contributors
13:43 [Info] Loaded plugin Rust:IO for Oxide v2.15.0 by playrust.io / dcode
13:43 [Info] Added '// Reference: System.Drawing' in plugin 'CopyPaste'
13:43 [Error] DeathNotesGui plugin requires missing dependency: PopupNotifications
13:43 [Info] Added '// Reference: Facepunch.Sqlite' in plugin 'ImageLibrary'
13:43 [Info] Added '// Reference: UnityEngine.UnityWebRequestModule' in plugin 'ImageLibrary'
13:43 [Warning] Ignored unnecessary '// Reference: Rust.Workshop' in plugin 'InstantCraft'
13:43 [Info] Added '// Reference: System.Drawing' in plugin 'SignArtist'
13:43 [Info] AdminRadar, AutoBroadcast, AutoDoors, AutomaticAuthorization, BetterChat, BGrade, BlueprintManager, BuildingBlocker, BuildingGrades, CarCommanderLite, Clans, ClanTags, ConnectMessages, CopyPaste, DeathNotes, FriendlyFire, GatherManager, Give, Godmode, HeliControl, HeliRide, HeliVote, HitIcon, ImageLibrary, InstantCraft, Kits, LoadingMessages, MagicLoot, MasterKey, NoEscape, NoFuelRequirements, NoGiveNotices, NoWorkbench, NTeleportation, PrivateMessage, Prod, QuickSmelt, RemoverTool, Rewards, ServerInfo, ServerRewards, SignArtist, Skins, SkipNightUI, StackSizeController, TankCommander, Telekinesis, Trade, UiPlus, Vanish, WellFed, WoundedScreams and ZLevelsRemastered were compiled successfully in 2897ms
13:43 [Warning] A plugin has polluted the global namespace by defining Oxide.Plugins.BasePlugin.BaseUiPlus
13:43 [Info] Loaded plugin Admin Radar v4.3.2 by nivex
13:43 [Info] Loaded plugin AutoBroadcast v1.0.8 by Wulf/lukespragg
13:43 [Info] Loaded plugin AutoDoors v3.1.2 by Wulf/lukespragg
13:43 [Info] Loaded plugin AutomaticAuthorization v0.2.3 by k1lly0u
13:43 [Info] Loaded plugin Better Chat v5.0.19 by LaserHydra
13:43 [Info] Loaded plugin BGrade v1.0.49 by Ryan
13:43 [Info] Loaded plugin Blueprint Manager v1.0.3 by Jacob
13:43 [Info] Loaded plugin BuildingBlocker v2.4.0 by Vlad-00003
13:43 [Info] Loaded plugin Building Grades v0.3.12 by bawNg / Nogrod
13:43 [Info] Loaded plugin CarCommanderLite v0.1.0 by k1lly0u
13:43 [Info] Loaded plugin Rust:IO Clans v1.7.3 by playrust.io / dcode
13:43 [Info] Loaded plugin Clan Tags v0.8.0 by GreenArrow
13:43 [Info] Loaded plugin ConnectMessages v1.1.9 by Spicy
13:43 [Info] Loaded plugin Copy Paste v3.6.7 by Reneb
13:43 [Info] Loaded plugin Death Notes v6.0.6 by LaserHydra
13:43 [Info] Loaded plugin FriendlyFire v1.6.0 by playrust.io / dcode
13:43 [Info] Loaded plugin Gather Manager v2.2.6 by Mughisi
13:43 [Info] Loaded plugin Give v3.0.1 by Wulf/lukespragg
13:43 [Info] Loaded plugin Godmode v4.1.1 by Wulf/lukespragg
13:43 [Info] Loaded plugin HeliControl v1.3.1 by Shady
13:43 [Warning] [Heli Ride] Chute plugin not found. To enable player parachutes when there helicopter dies, install Chute plugin!
13:43 [Warning] [Heli Ride] Vanish plugin not found. Player will be visable and UseAutoVanish will be turned off
13:43 [Info] Loaded plugin Heli Ride v1.1.13 by ColonBlow
13:43 [Info] Loaded plugin HeliVote v0.1.32 by k1lly0u
13:43 [Info] Loaded plugin HitIcon v1.5.9 by serezhadelaet
13:43 [Info] Loaded plugin Image Library v2.0.44 by Absolut & K1lly0u
13:43 [Info] Loaded plugin InstantCraft v1.2.3 by Vlad-00003
13:43 [Info] Loaded plugin Kits v3.2.93 by Reneb
13:43 [Info] Loaded plugin Loading Messages v1.0.4 by VVoid
13:43 [Info] Loaded plugin MagicLoot v0.1.24 by Norn / Kunazai
13:43 [Info] Loaded plugin Master Key v0.7.3 by Tori1157
13:43 [Info] Loaded plugin NoEscape v2.0.91 by Calytic
13:43 [Info] Loaded plugin NoFuelRequirements v1.3.6 by k1lly0u
13:43 [Info] Loaded plugin NoGiveNotices v0.1.0 by Wulf/lukespragg
13:43 [Info] Loaded plugin NoWorkbench v0.1.50 by k1lly0u
13:43 [Info] Loaded plugin NTeleportation v1.0.26 by Nogrod
13:43 [Info] Loaded plugin PrivateMessage v2.0.2 by Nogrod
13:43 [Info] Loaded plugin Prod v2.2.9 by Reneb
13:43 [Info] Loaded plugin QuickSmelt v4.0.5 by Wulf/Jake-Rich
13:43 [Info] Loaded plugin RemoverTool v4.2.10 by Reneb/Fuji
13:43 [Info] Loaded plugin Rewards v1.3.14 by Tarek
13:43 [Info] Loaded plugin ServerInfo v0.5.3 by Fujikura/baton
13:43 [Info] Loaded plugin ServerRewards v0.4.67 by k1lly0u
13:43 [Info] Loaded plugin Sign Artist v1.1.2 by Mughisi
13:43 [Info] Loaded plugin Skins v1.2.9 by MalkoR
13:43 [Info] Loaded plugin SkipNightUI v0.1.2 by k1lly0u
13:43 [Info] Loaded plugin Stack Size Controller v1.9.9 by Canopy Sheep
13:43 [Info] Loaded plugin Tank Commander v0.1.6 by k1lly0u
13:43 [Info] Loaded plugin Telekinesis v2.0.8 by redBDGR
13:43 [Info] Loaded plugin Trade v1.2.1 by Calytic/rustservers.io
13:43 [Info] Loaded plugin UiPlus v1.1.3 by RedKenrok
13:43 [Info] Loaded plugin Vanish v0.7.0 by nivex
13:43 [Info] Loaded plugin WellFed v1.1.5 by ColonBlow
13:43 [Info] Loaded plugin Wounded Screams v2.1.4 by Skipcast
13:43 [Info] Loaded plugin ZLevelsRemastered v2.9.5 by FuJiCuRa
13:43 [Info] [InstantCraft] Pulled 1451 skins.
13:45 [Info] Loading Oxide Core v2.0.3867...​

As you can see, the last line is Oxide restarting after the server crashed. So I think it crashed on InstantCraft pulling skins?

Am I right in assuming this or should I check something else?

In one of the other logs (not in the Oxide folder) I find this

[Better Chat] Sebastian118811: hello
 
(Filename: C:\buildslave\unity\build\Runtime/Export/Debug.bindings.h Line: 43)

Khan[2493553/76561198817811431] has entered the game
 
(Filename: C:\buildslave\unity\build\Runtime/Export/Debug.bindings.h Line: 43)

[Better Chat] [Admin] [God] Professor Goldsteinberg: Hello
 
(Filename: C:\buildslave\unity\build\Runtime/Export/Debug.bindings.h Line: 43)

[Death Notes] [BATZ] DaDi shot aanillo30 hellcase.com using their LR-300 Assault Rifle over a distance of 0.9 meters.
 
(Filename: C:\buildslave\unity\build\Runtime/Export/Debug.bindings.h Line: 43)

[Rewards] [BATZ] DaDi got 1500 for killing aanillo30 hellcase.com
 
(Filename: C:\buildslave\unity\build\Runtime/Export/Debug.bindings.h Line: 43)

aanillo30 hellcase.com[2707056/76561198144110233] was killed by [BATZ] DaDi[571687/76561198095460350]
DynamicHeapAllocator allocation probe 1 failed - Could not get memory for large allocation 2118400.
DynamicHeapAllocator allocation probe 2 failed - Could not get memory for large allocation 2118400.​

This could explain something.. not getting enough memory.

But how do I make sure this doesnt happen again? Because this isnt'the first time the server crashed.

If the server ran out of memory, then it would need more memory. 

Do you think thats the reason for the crash? How about what Oxide says about the InstaCraft?

Running out of memory will the crash the server, yes. The "warning" from Oxide is nothing, just a warning while the plugin was being compiled.