The Startup tab decides what actually runs when you press start. Most of it you set once and never look at again — but two fields are behind a large share of servers that will not boot.
The fields that matter
| Field | What it controls |
|---|---|
| Server jar file | Which file Java runs. Must match a file that exists, exactly, including the version. |
| Minecraft version | Which build the installer fetches on reinstall. Changing it alone does not change a running server. |
| Server type | Paper, Spigot, Fabric, Forge, Vanilla. Changing it changes what plugins and mods work. |
| Startup command | The full Java line. You rarely touch this and should be careful when you do. |
If the field says server.jar and your file is called paper-1.21.1-40.jar, the server dies immediately with «Unable to access jarfile». The panel does not check this for you, and the error is at the very top of the console, above everything else.
Changing your server version safely
- Back up first
A version change is not always reversible. A world opened by a newer version cannot be opened by an older one.
- Change the version field, then reinstall
The version field tells the installer what to fetch. Nothing changes until it runs.
- Check the jar filename still matches
A new version usually means a new filename. This is where most upgrades fail.
- Update your plugins in the same session
Plugins for the old version often will not load on the new one. Doing both at once means one round of testing, not two.
Java memory arguments are set for the plan you are on. Raising them past what you have does not give you more memory; it gives you a server that gets killed the moment it tries to use what it was promised.
The console shows the version you expected in its startup banner, and no jarfile error.