AccessWidenerFormatException: Namespace (named) does not match current runtime namespace (intermediary)

What the log looks like

net\.fabricmc\.loader\.impl\.lib\.accesswidener\.AccessWidenerFormatException: line \d+: Namespace \(named\) does not match current runtime namespace \(intermediary\)

How to fix it

1. Check the crash log for the line 'Failed to read accessWidener file from mod <modname>' to identify the broken mod.
2. Open your Minecraft instance's 'mods' folder and find the .jar file belonging to that mod.
3. If the filename mentions 'neoforge' or 'forge' (for example, 'iris-neoforge-1.8.10+mc1.21.5.jar'), delete it and download the Fabric version for your exact Minecraft version.
4. If the filename ends in '-dev.jar' or '-sources.jar', delete it and download the regular release .jar from Modrinth or CurseForge.
5. If you compiled the mod yourself from GitHub source code, run './gradlew build' (or 'gradlew.bat build' on Windows) so Loom runs 'remapJar', then copy only the main .jar from 'build/libs/' without '-dev' or '-sources' in its name.

Check your own log