You might try this. Fill in Weird Story location waymark with fictitious information. Then edit it filling in only a couple pieces of correct information and save. Keep doing this until you add something that causes the error. This will help identify what is causing the error.
Edited to Add:
I have just run a test on the category and I as able to enter and save a test waymark and then went back in and edited it. I did not get the error screen.