Looks like something conflicts with Quark and I cant really get it.... It spams this error and you cant interact with anything. Its like every mob is moving and working but you cant interact anything. Then your server crashs after 2 min.
There is no performance related mod on server.

Failed to save chunk [28, -42]
java.util.concurrent.CompletionException: java.lang.ClassCastException: class vazkii.quark.content.building.tile.VariantChestTileEntity cannot be cast to class net.minecraft.entity.Entity (vazkii.quark.content.building.tile.VariantChestTileEntity and net.minecraft.entity.Entity are in unnamed module of loader cpw.mods.modlauncher.TransformingClassLoader @405603b6)
at java.util.concurrent.CompletableFuture.encodeThrowable(CompletableFuture.java:314) ~[?:?]
at java.util.concurrent.CompletableFuture.completeThrowable(CompletableFuture.java:319) ~[?:?]
at java.util.concurrent.CompletableFuture$UniAccept.tryFire(CompletableFuture.java:718) ~[?:?]
at java.util.concurrent.CompletableFuture$Completion.run(CompletableFuture.java:478) ~[?:?]
at net.minecraft.world.server.ChunkManager.func_223155_b(ChunkManager.java:416) ~[?:?]
at net.minecraft.world.server.ChunkManager.func_219204_a(ChunkManager.java:394) ~[?:?]
at net.minecraft.world.server.ServerChunkProvider.func_217207_a(ServerChunkProvider.java:388) ~[?:?]
at net.minecraft.world.server.ServerWorld.func_72835_b(ServerWorld.java:489) ~[?:?]
at net.minecraft.server.MinecraftServer.func_71190_q(MinecraftServer.java:1093) ~[?:?]
at net.minecraft.server.dedicated.DedicatedServer.func_71190_q(DedicatedServer.java:334) ~[?:?]
at net.minecraft.server.MinecraftServer.func_71217_p(MinecraftServer.java:1006) ~[?:?]
at net.minecraft.server.MinecraftServer.func_240802_v_(MinecraftServer.java:821) ~[?:?]
at net.minecraft.server.MinecraftServer.func_240783_a_(MinecraftServer.java:269) ~[?:?]
at java.lang.Thread.run(Thread.java:834) [?:?]
Caused by: java.lang.ClassCastException: class vazkii.quark.content.building.tile.VariantChestTileEntity cannot be cast to class net.minecraft.entity.Entity (vazkii.quark.content.building.tile.VariantChestTileEntity and net.minecraft.entity.Entity are in unnamed module of loader cpw.mods.modlauncher.TransformingClassLoader @405603b6)
at net.minecraft.world.server.ServerWorld.redirect$znm000$onEntityRemoved(ServerWorld.java:3931) ~[?:?]
at net.minecraft.world.server.ServerWorld.func_217466_a(ServerWorld.java:1139) ~[?:?]
at net.minecraft.world.server.ChunkManager.func_219185_a(ChunkManager.java:437) ~[?:?]
at java.util.concurrent.CompletableFuture$UniAccept.tryFire(CompletableFuture.java:714) ~[?:?]
... 11 more

I have zero idea what could be causing this at all. It looks like some mod is doing something really weird because the log doesn't point to anything that would be relevant.