A modpack is a curated set of mods with a specific loader version and specific configuration. The important part is that the server and every client run the same pack build — not just the same pack.
What to do
- Note the pack name and the exact build number
«All The Mods 9» is not enough; packs update, and build 0.2.60 and 0.2.61 are different mod lists. Write down the version you intend to run.
- Install the server pack, not the client pack
Most packs publish a separate server download. It contains the same mods minus the client-only ones, plus a start script. Installing the client pack on a server usually fails on rendering mods.
- Give it enough memory before the first start
Packs load everything at boot. A pack that documents 8 GB will not start in 6, and the failure looks like a crash rather than a shortage.
- Have players install the same build from the launcher
CurseForge, Prism or the pack's own launcher. Same pack, same build number as the server.
The console reaches Done — this takes several minutes on a large pack — and a player running the same build joins without a mod mismatch.
The first start of a big pack is slow enough that people assume it has hung. Watch the console rather than the clock: as long as lines are still appearing, it is working.