Most Fabric Language Kotlin problems are dependency problems rather than gameplay bugs. The fastest solution is usually to identify which mod requested the library and what exact version range it accepts.
Missing dependency
If Fabric Loader says fabric-language-kotlin is missing, verify that a compiled JAR exists in the active instance mods folder. A GitHub source ZIP will not be loaded as a mod.
Also confirm that your launcher has not switched to a different instance with a different mods directory.
Incompatible version
Read the requested minimum, maximum, or exact range shown in the error. Replace the installed library with a build that fits the range, or update the dependent mod if it only supports an older library.
Avoid keeping several versions in the same folder.
Class loading or Kotlin errors
Errors naming Kotlin classes can result from a missing or incompatible language library, but they can also come from a broken dependent mod. Reproduce the issue with only the affected mod and its required dependencies.
Server starts locally but not remotely
Compare server and local Java versions, Fabric Loader versions, and the mods folder contents. Hosting panels sometimes use a different launch profile than the one you tested.
Upload JARs in binary form and confirm the hosting panel did not extract them.