Instead to use the 404 page provided by the theme, it seems index.php is included a new time.
Example: https://tools51.nasqueron.org/404
Fatal error: Cannot redeclare force_bare_display() (previously declared in /usr/home/dereckson/dev/nasqueron/tools/repo/index.php:71) in /usr/home/dereckson/dev/nasqueron/tools/repo/index.php on line 75
Note: after T700 resolution, error will be:
Fatal error: Cannot redeclare get_username() (previously declared in /usr/home/dereckson/dev/nasqueron/tools/repo/includes/core.php:43) in /usr/home/dereckson/dev/nasqueron/tools/repo/includes/core.php on line 49
I can't repro anymore this issue, when I investigated to a header/body write order issue, fixed in D1169.