Thanks.
I deleted the error_log in the folder of the problematic site. Then renamed my functions.php.bk back to functions.php to recreate the crashing. When I tried to logout of the site, the blank screen happened as expected, though there was no new error_log created.
Then I created an empty error_log with the proper permissions, as I thought that this might be the problem… Tested once again logging out, I got the blank screen, but still nothing in the error_log.
So I guess this way is a dead end.. no?
By the way, when the blank screen happens, the URL in the browser shows something like this:
https://www.mysite.com/wp-login.php?action=logout&_wpnonce=7cf8f6454e
while when all goes well, the URL after logging out is:
https://www.mysite.com/wp-login.php?loggedout=true
Can we assume anything from that?