How to change TML page slugs?

Visit your admin dashboard, TML > Pages. Edit them as you would any other post.

When I tried that, the login page wouldn’t load.

If I rename the slug to “login”, browser gives “This webpage has a redirect loop”.
If I rename the page to anything else, I get 404

Do you happen to have more than one login page?

No, only the one on this site.

I’ve tested on a vanilla install and it’s exhibiting the same behaviour.

Also I’ve noticed that “Enable Custom Passwords” is killing form output on the Registration page.

Which version is this?

Hm, it seems it’s 6.4a. I wouldn’t normally run an alpha, but I’m pretty certain it came via auto update notice in WP admin.

Should I roll back to the current stable?

Yes, definitely!