Use a custom JAR or startup command

Keep your entry point and launch command consistent.

The actual server runs in /server in the file manager. Startup commands are configured under Versions, where available.

Step by step

Back up and stop the server before replacing its executable.
Upload the intended JAR or startup files into /server, not /backups.
Check that the startup command references the actual filename with the exact letter case.
For an installer-based distribution, keep its generated libraries and launch script rather than treating the installer as a normal runtime JAR.
Start once and inspect the console.

Check the result

Unable to access jarfile usually points to a missing filename, wrong location or mismatched command.

Important

Only run trusted code. Version changes, reinstallation or a Fix Tool JAR repair can replace custom runtime files.