I keep getting the "Login to Continue" dialog appearing, even though I'm logged in, and can just close the dialog and carry on. I tried clearing cache and stache to see if that helps, but it doesn't.

I found this old-ish thread, which seemed to be the same problem:

https://statamic.com/forum/3377-client-keeps-getting-booted-out-of-control-panel

But wary of creating a .env file on a live site if it causes problems with any forms. In particular, I have a password protected part of the site. Is there any more recent information on this issue?