If you use the Panel, it would have sluggified your page folder names. I’m not 100% sure if dots are allowed at all, but the issue is definitely caused by the underscores. If I were you, I’d remove both dots and underscores (and any other weird characters) from the page slug.
The underscore is used by Kirby to denominate the numbering scheme for listed pages.
Can you open the bla page without metadata.json part?
The 404 error just got weirder. Nothing apart from the homepage loads now and the images on the homepage are blank (I am using the starter kit). Just to make sure I copied over a brand new starter kit and the issue persists. I swear it was working just a few hours ago. Not sure what is wrong here. Need to investigate.