If you try to upgrade the world using the original version of the game without the fabric, the error message is simpler, but the reason is the same, so I think it's a Minecraft issue.
[00:17:33] [Worker-Main-20/INFO]: Starting upgrade for world "world" [00:17:33] [Worker-Main-20/INFO]: Upgrading progress: 2% [00:17:34] [Worker-Main-20/ERROR]: Failed to write to /level.dat: java.nio.file.FileSystemException: /home/acdd233/.var/app/org.prismlauncher.PrismLauncher/data/PrismLauncher/instances/26.1/minecraft/saves/world/filefix/cow/tmp_9: Invalid argument [00:17:52] [Worker-Main-10/INFO]: Starting upgrade for world "world" [00:17:52] [Worker-Main-10/INFO]: Upgrading progress: 2% [00:17:52] [Worker-Main-10/ERROR]: Failed to write to /level.dat: java.nio.file.FileSystemException: /home/acdd233/.var/app/org.prismlauncher.PrismLauncher/data/PrismLauncher/instances/26.1/minecraft/saves/world/filefix/cow/tmp_9: Invalid argument
If you try to upgrade the world using the original version of the game without the fabric, the error message is simpler, but the reason is the same, so I think it's a Minecraft issue.