This error on Forge servers usually means the Forge startup files (the unix_args.txt in the libraries folder) are broken or don't match the configured version.
How to fix it
- Note your exact Minecraft and Forge versions.
- In the panel use the MC Version and select Forge with that version.
- Let it reinstall the Forge files correctly.
- In the Startup tab, make sure the JAR parameter is at its default (don't edit it by hand).
- Start and check the console.
If it was caused by uploading a Forge version manually, let the MC Version handle it: it installs the right libraries for you.
Still failing? Open a ticket with your Forge version and the console output.
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