Name and link of Custom World (or did you made it yourself? Perhaps its not EP/Isla Paradiso compatible?

I'm currently in the process of making it, it's not even five weeks old. I don't see why it wouldn't be compatible. My first world attempt that is only a few weeks older than this current one works just fine with resorts.
The resorts work in a fresh new world (of the world with the problem), but after the first save and exit, upon reloading the world and playing in live-mode, the error starts.

I have uploaded the exact script error according to the guide How To Upload if anything I failed to mention is relevant.

You have a ResortManager instance that has lost it's ties to the lot it's controlling. See if setting the lot type to something else and then back again stops the issue.

Changing the lot to a different type stopped the script errors, but when I changed it back to a resort the errors continued and I could not interact with the lot once again.

Well that's progress. Try sending the lot to the bin, bulldozing and replacing.

Nope :(

Stubborn lot data is stubborn. Do you have any other lots in the world the lot will fit on to see if it errors there?

I've tried just changing a random lot to a resort to see if it was lot specific. Unfortunately the problem isn't. It happens on any lot that's a resort-type after save-exiting the world and reopening, though the script errors seem to say the exact same thing...

Interesting. And you have resorts in other worlds that don't produce the error?