For our custom tooling, we modify the log format using -Dlog4j.configurationFile=log4j2.xml. However, Fabric crashes on startup if this option is given. (The vanilla server does not crash when using the same configuration file.) We had to disable this option for modded worlds for the time being, but this also breaks some features that work by reading the log.

This issue would probably be better asked on the Fabric Loader repository. Fabric api should be having no effect on this at all.

Can someone close this on this end please?