java.lang.IllegalStateException: Rendersystem called from wrong thread

What the log looks like

java\.lang\.IllegalStateException: Rendersystem called from wrong thread
Exception caught during firing event: Rendersystem called from wrong thread
com\.mojang\.blaze3d\.systems\.RenderSystem\.assertOnRenderThread

How to fix it

1. Look directly below 'Rendersystem called from wrong thread' in your log: if there is an 'Index:' number above a 'Listeners:' list, find the line matching that exact index number to identify the mod making the illegal render call.
2. If the log line starts with '[REI-ReloadPlugins/ERROR]', open Roughly Enough Items (REI) config settings and disable asynchronous plugin/search reloading so item tooltips are queried on the main client thread.
3. If the stack trace mentions 'ConfigWatcher' and 'NeoForgeClientConfig.onFileChange' on Minecraft 1.21.5, update NeoForge to 21.5.19-beta or newer and avoid editing client config files while the game is running.
4. Update the mod named in the stack trace (or at the matching listener index) and your mod loader to their latest versions, or temporarily remove that mod if it continues crashing.

Check your own log