I've fired up ZED and have been messing around with mapping again. It's amazing how fast it all comes back.

However, I can't seem to test my levels. Trying to save JLK and test doesn't launch JK at all, and when I GOB my map and run JK I just get the cannot load level error. I've filled out the episode information so I'm not sure where I'm going wrong here.

Are you hitting "update" after filling out episode info? The FAQ says hit Update instead of Save but i always hit Update then Save and it seems to work. Episode editor points to the right jkl?

I recently had a problem like this after opening up some old projects from many years ago wanting to work on them some more. I started a whole new project and just tried to make myself a single player map in a dflt box then troubleshot from there. It was definitely something in my episode editor/startup cogs and I believe i fixed it by clearing everything and starting over on the episode editor.

Other common issue is that the walkplayer isn't there or isn't in a sector. Did you check the consistency checker? To fix it, you just move the walkplayer, let go of mouse, then move it back.