This is an error that occurred while developing a personal app with Rails. As a memorandum, I will write from the discovery of the error to its resolution.
When I access the sign-up page on my smartphone (OS: iOS, browser: Google Chrome, Safari), the layout of the page is displayed for a moment, and then the error "This page cannot be opened" occurs.
However, if you access the URL directly, the page will be displayed normally. (This ended up being a mystery ...)
When I searched for other pages that gave the same error, it also occurred on the profile edit page.
What the sign-up page and the profile edit page have in common is that they have a form for entering user information.
If there are multiple autofocus: true forms in one screen, it seems that an error occurs when browsing on a smartphone. So I left autofocus: true only on the very first input form and removed it from the rest of the forms and the error went away. I overlooked it because there was no error when browsing on a PC ... It is good to increase the form by copying and pasting, but you have to maintain this area well.