This happened to me again today (suddenly getting the install screen), first time for this particular WordPress site, though, but it has happened to me on others. So I FTP’d into the site, renamed .htaccess so it wouldn’t function, renamed install.php so that it wouldn’t function either (later WordFence alerted me that it found that renamed “unauthorized file” in my WP setup and it should be removed—cool!) and was then able to log in to my WP site OK. Then I went into Settings>Permalinks and just did a save there to re-establish an .htaccess file. When this problem first occurred, I had been doing a lot of small edits on many custom posts, so the idea of stack overflow mentioned earlier in this thread made me think that the unusually large (a few dozen) number of edits made in the span of a few minutes might have had something to do with it. Still, it’s a mystery to me of what actually triggered it to happen.
