Check the server mod requirements
Review the dependency declarations of the mods installed on your server. When a server mod needs Placebo, the library must be available to that server process. Installing it on your personal client does not change the remote server's files.
Stop before changing jars
Back up the server world, configs and current mod list, then stop the server normally. Put the compatible main jar in the server mods folder, remove obsolete duplicates and restart using the intended loader. Read the startup log before inviting players back.
Share a tested player pack
Give players the modpack version that matches your tested server setup, including dependencies required on their side. If a connection fails, compare exact versions and logs rather than assuming every library must always be identical regardless of the dependent mods' requirements.