I get an “Item doesn’t exist” error after logging in

I’m having trouble using a Colorlib Login Customizer page to log in. The following picture shows the page I’ve been using to make the login page.

When I use the page I made with the Colorlib Login Customizer, it logs me in, but takes me to a page that has the message “You attempted to edit an item that doesn’t exist. Perhaps it was deleted?” (If other users get that message, they might not know that they were logged in.) Sometimes, when I log in as a user that isn’t an administrator, I get another error message that sounds to me like it is trying to take me to a page that only administrators can use, because it says that I need a higher level of permission. Can it remember where I was before I logged out, even if I was an administrator before I logged out, and I’m not trying to log in as an administrator? If it is remembering where I was before I logged out (whether I was an administrator before I logged out or not), is there a way to stop it from remembering? Could this be a problem with the form that I use to log out? Any help would be appreciated.

Also, sometimes when I log out it (1) takes me to the page that I made with the Colorlib Login Customizer, and at other times it (2) brings me to my home page (where there is a different login form). How does it decide which of those two to do? Can I tell it which of those two to do?

Thank you.