Error message when I try to copy a class or add a new blank class
 

I get this when I try to use New (Blank) Class or New (Copy) Class:

https://imgur.com/a/s3xwW0L

When that happens. I am unable to click on or edit anything.

Anyone know how to fix this?

That's a known error with HLC Pathfinder. It doesn't mean you did anything wrong - it's like a memory overload. Once it happens, you have to exit HL and go back in. It should occur less if you use HL 64-bit, instead of 32.

Okay... So now I'm getting THIS error message when I try to load Pathfinder full stop: https://imgur.com/mJrp33N

What causes it and how do I fix it?

Once HL encounters an error, it won't load any user files.
You have a file called "blag.user" with something using the code arLUN? for its ID. The ID can't have a question mark. I think you copied something else, and HL gave it a temp ID that you haven't changed to a proper ID.
Load the user file in the editor, find the item and delete it or give it a proper ID.