At some point my admin/pages section became dumb - I cannot open any page for editing. When I looked into DevTools/Network I noticed that the response begins with `<?php<?php` as in:

<?php<?php{"Content":"<div id=\"pages-controller-cms-content\"

Tried to undo my operations, but to no avail - is that some kind of a known problem?
v3.1.3

Have you been editing any of the core CMS or Framework files? (which is not a good idea)

Oh, no, never-ever!
Actually, I have solved it a moment ago - the problem was in empty _config.php files that contained <?php only. When I addded an empty comment below it helped.
Thanks for reply though!

the old php trailing spaces issue... yeah, if you have a _config.php with nothing in it it pays to add the extra line underneath.

I wonder if you want to change the title of your post to represent this actual problem and add a [solved] to the front. That way it is useful to others.