This error message seems to indicate that some other process is messing with your Fabric Loader. One possible cause could be another Minecraft instance running in the background and trying to access the same files.
Try checking your task manager for any Minecraft- or Java-related processes and stop them. If you don't see any other Minecraft processes or can't figure out the culprit, a simple restart of your computer should clear up any mysterious processes that might be causing this error.
Hope this helps! Let me know if you encounter any other issues.