The reason it’s failing is FPackageName::ParseExportTextPath() is returning false. That means it couldn’t parse the input path to the package. The error says the input path is an zero-length string. Beyond that it’s hard to tell. Maybe with more disclosure I’d have a better idea.
If that’s not possible (and it sounds like it isn’t) then I suggest you try and figure out why the input path is invalid. I’d check what’s being used with the LoadFrontEndMap call.
You could build a package for Debug, run the packaged game in the debugger, and look at the input path (and call stack) when it fails. There might be a thread there to pull on.