Hello everyone,
I wanted to share a simple workaround that helped in this case, and hopefully it may help others experiencing a similar issue as well.
Error Case
→ Epic Games Launcher > Unreal Engine tab > Fab Library > Install to Engine
→ IS-0009 error occurs
Workaround
→ In this error state, completely restart the Epic Games Launcher, then install the plugin from:
Epic Games Launcher > Fab tab > Plugins > Install Plugin
If anyone else has experienced a similar issue or found other solutions, please feel free to share them in the comments.