Server crash while starting

Hello. I recently updated my server to 1.13, but later downgraded it to 1.12. After the downgrade, the server stops while starting. I found this message in the console:

[15:57:20] [Server thread/WARN]: Could not set level chunk heightmap, array length is 0 instead of 256
[15:57:20] [Server thread/ERROR]: Encountered an unexpected exception
java.lang.IllegalArgumentException: ChunkNibbleArrays should be 2048 bytes not: 0
at net.minecraft.server.v1_12_R1.NibbleArray.<init>(SourceFile:16) ~inecraft_server.jar:git-Bukkit-9a1fc1e]
at net.minecraft.server.v1_12_R1.ChunkRegionLoader.a(ChunkRegionLoader.java:365) ~inecraft_server.jar:git-Bukkit-9a1fc1e]
at net.minecraft.server.v1_12_R1.ChunkRegionLoader.a(ChunkRegionLoader.java:85) ~inecraft_server.jar:git-Bukkit-9a1fc1e]
at net.minecraft.server.v1_12_R1.ChunkRegionLoader.loadChunk(ChunkRegionLoader.java:63) ~inecraft_server.jar:git-Bukkit-9a1fc1e]
at org.bukkit.craftbukkit.v1_12_R1.chunkio.ChunkIOProvider.callStage1(ChunkIOProvider.java:21) ~inecraft_server.jar:git-Bukkit-9a1fc1e]
at org.bukkit.craftbukkit.v1_12_R1.chunkio.ChunkIOProvider.callStage1(ChunkIOProvider.java:1) ~inecraft_server.jar:git-Bukkit-9a1fc1e]
at org.bukkit.craftbukkit.v1_12_R1.util.AsynchronousExecutor.skipQueue(AsynchronousExecutor.java:336) ~inecraft_server.jar:git-Bukkit-9a1fc1e]
at org.bukkit.craftbukkit.v1_12_R1.util.AsynchronousExecutor.getSkipQueue(AsynchronousExecutor.java:295) ~inecraft_server.jar:git-Bukkit-9a1fc1e]
at org.bukkit.craftbukkit.v1_12_R1.chunkio.ChunkIOExecutor.syncChunkLoad(ChunkIOExecutor.java:16) ~inecraft_server.jar:git-Bukkit-9a1fc1e]
at net.minecraft.server.v1_12_R1.ChunkProviderServer.getChunkAt(ChunkProviderServer.java:137) ~inecraft_server.jar:git-Bukkit-9a1fc1e]
at net.minecraft.server.v1_12_R1.ChunkProviderServer.getChunkAt(ChunkProviderServer.java:120) ~inecraft_server.jar:git-Bukkit-9a1fc1e]
at net.minecraft.server.v1_12_R1.ChunkProviderServer.getChunkAt(ChunkProviderServer.java:116) ~inecraft_server.jar:git-Bukkit-9a1fc1e]
at net.minecraft.server.v1_12_R1.MinecraftServer.l(MinecraftServer.java:363) ~inecraft_server.jar:git-Bukkit-9a1fc1e]
at net.minecraft.server.v1_12_R1.MinecraftServer.a(MinecraftServer.java:327) ~inecraft_server.jar:git-Bukkit-9a1fc1e]
at net.minecraft.server.v1_12_R1.DedicatedServer.init(DedicatedServer.java:248) ~inecraft_server.jar:git-Bukkit-9a1fc1e]
at net.minecraft.server.v1_12_R1.MinecraftServer.run(MinecraftServer.java:521) inecraft_server.jar:git-Bukkit-9a1fc1e]
at java.lang.Thread.run(Thread.java:748) [?:1.8.0_144]
[15:57:20] [Server thread/ERROR]: This crash report has been saved to: /aternos/server/./crash-reports/crash-2018-07-31_15.57.20-server.txt
[15:57:20] [Server thread/INFO]: Stopping server

Sorry for the console message being grey, you can copy it to Notepad in order to read it better

Quote from SLINIcraftet204

hey i have another option use ithere this is my tip

Already tried, doesn't detect anything

I now know why:

<<The world save format has changed; worlds older than 1.13 cannot be opened with versions from 1.13 or later without permanently and irreversibly upgrading them to the new format, making it impossible to open them in earlier versions>>

I guess. I tried with a backup but it doesn't work, I'll need to play in 1.13