I have the same problem with Hide Backend since last update 5.4.5. Lets hope iThemes fix it quickly. It don′t work anymore.
PS (EDITED)
I have fix the problem myself.
Its very easy to fix.
I don′t go into details, just UI simple fix, we need to save first Hide Backend with default plugin options enabling Hide Backend, after that we can personalize the url as usual before 5.4.5.
There is a problem with a popup that I think is suppose to say something like “saved!” after we change something but it show only white. Anyway that is not Hide Backend related, just a bug to fix since last update.