Java was given a filename and could not find it. This is not a Minecraft error at all — it happens before Minecraft starts — which is why the console shows almost nothing else.
What you are seeing
Error: Unable to access jarfile server.jar- The console produces one line and stops.
- It started after changing version, uploading files or editing the startup command.
Why it happens
The startup command names a jar file. If that file does not exist, has a different name, or the download that should have produced it failed, Java stops immediately.
What to do
- Compare the name in the startup command with the file manager
They have to match exactly, including capitalisation and the
.jarending.Server.jarandserver.jarare two different files. - Check the file is actually there and not empty
A failed download can leave a file of 0 bytes. If the size looks wrong, delete it and let the server fetch it again on the next start.
- Reinstall if the jar is missing entirely
Reinstalling the server software fetches a fresh jar. Your world and configuration files are separate from it — but take a backup first anyway.
The console gets past this line and starts printing Minecraft's own startup output.
If you uploaded a custom jar, check it finished uploading. A connection dropped halfway leaves a truncated file that exists but cannot be run.