I had this exact same problem, and I assumed it was a Java or a mod pack problem, which it wasn't. After many hours I found out its a forge problem.
So, heres how I fixed my server.
1. Downloaded the latest forge installer for my version (1.16.5-36.2.30 in my case)
2. Ran the installer, told it to 'Install Server' and selected my minecraft server folder.
3. Once it had installed the latest version of forge, editted the line in the start.bat file to use the latest version. Changed from forge-1.16.5-36.2.23.jar to forge-1.16.5-36.2.30.jar in my case.
4. Ran start.bat, everything worked.
Hope this helps someone else.