Although it maybe an effective method, spending hours loading and unloading the server, moving mods one at a time into a folder, Is not a good strategy unless every other method has been exhausted.
Also he said mod. Is it possible he means mod or plugin, it is singular so it sounds like he has a specific one in mind. Which means we can narrow the problem possibly to a forge version issue, mod conflict, or dependency issue which can be easily discovered if he goes into his server’s folder and looks inside the /crash-reports folder for the latest error report which he can upload to hastebin if he can’t understand and post here for community support, or on the forum /thread of whatever the mod/plugin of whatever is causing the issue.
It is better to ask a man for information before sending him to his own mental demise. Tons of repetitive, inefficient, and eye straining work can lead anyone to just throw in the towel. Sponge is a community that likes to enable people not discourage them by making things more difficult than they should be.
Don’t solve something if you don’t know what the problem is, find the problem first.