We seem to be having the same problem when trying to view/preview mobile apps in a web browser (editing works fine). Is yours a Retool Mobile app, @Ori ?
After a bit of trial and error I noticed pressing the preview/play button in the upper-right of the editor doesn't work.
Comparing the two URL's - the one that was failing to load contained the App ID, while the one that loaded properly had the app name. Copy/pasting the working URL and launching it allowed our users to access the app.
As far as I know this just started happening this morning for us.