Basically we play a bit on the Server and after a while our console gets spammed by the following:

[23:24:01] [Server thread/ERROR]: Couldn't save chunk; already in use by another instance of Minecraft?[23:24:01] [Server thread/ERROR]: Couldn't save chunk; already in use by another instance of Minecraft?bhj: Failed to check session lock, aborting at cob.b(SourceFile:115) ~[minecraft_server.jar:?]
at vg.v(SourceFile:1205) ~[minecraft_server.jar:?]
at us.a(SourceFile:580) ~[minecraft_server.jar:?]
at us.a(SourceFile:387) ~[minecraft_server.jar:?] at java.util.concurrent.CompletableFuture.uniAccept(CompletableFuture.java:656) [?:?] at java.util.concurrent.CompletableFuture$UniAccept.tryFire(CompletableFuture.java:632) [?:?]
at java.util.concurrent.CompletableFuture$Completion.run(CompletableFuture.java:442) [?:?] at agh.h(SourceFile:135) [minecraft_server.jar:?]
at agh.p(SourceFile:114) [minecraft_server.jar:?] at ve$a.p(SourceFile:438) [minecraft_server.jar:?] at ve.d(SourceFile:194) [minecraft_server.jar:?] at net.minecraft.server.MinecraftServer.aW(SourceFile:708) [minecraft_server.jar:?] at net.minecraft.server.MinecraftServer.p(SourceFile:696) [minecraft_server.jar:?]
at agh.bf(SourceFile:99) [minecraft_server.jar:?] at net.minecraft.server.MinecraftServer.o(SourceFile:680) [minecraft_server.jar:?] at net.minecraft.server.MinecraftServer.run(SourceFile:636) [minecraft_server.jar:?] at java.lang.Thread.run(Thread.java:748) [?:?]

And by saying SPAMMED I really mean it. This repeats like every second making a 10minute log around 25MB. I already deleted the session.lock file but that did not seem to help. Any other Ideas? After a restart the Server gets reseted to the point where the error began in the console. Inventories stay saved but everything in the world is reset.