If you open a UEFN project that has Verse code that previously compiled successfully but currently has Verse code that will not compile in the current version of the compiler, you may run into a case where you will encounter errors unrelated to your code. This may also persist across opening different projects until you restart the editor.
This will be fixed in an upcoming UEFN release. However, to workaround the issue for now, please delete the snapshot data for your project and restart UEFN. This folder should be located at: %localappdata%\UnrealEditorFortnite\Saved\VerseSnapshot\
You can also fix this by making sure that your Verse code compiles without any errors, before attempting to open it in UEFN.