Step 1: create or duplicate a 1.21.1 profile
Do not convert your only working modpack in place. Create a fresh 1.21.1 NeoForge profile or duplicate an existing one so rollback is immediate.
Launch once with the loader alone. That confirms the base profile starts before you add Quark.
Step 2: add Quark and Zeta together
Place compatible release files into the profile’s mods folder. Keep the two files together during updates because Quark and Zeta compatibility is closely related.
Avoid combining a 1.21.1 Quark file with a 1.20.1 dependency or a Forge build with NeoForge. Similar filenames do not imply runtime compatibility.
Step 3: test before opening important worlds
Start the game, verify the mod list and create a temporary world. Check menus, recipes and a few modules before loading a long-term save.
If startup fails, read the earliest relevant dependency or loader error rather than only the final “game crashed” line.
Step 4: update carefully
When moving to a newer 4.1 build, read the change scope, back up the instance and update dependencies if required. Keep a copy of the previous working set.
For a server, update server and client expectations together so players are not connecting with incompatible mod sets.