[phpBB Debug] PHP Warning: in file [ROOT]/phpbb/db/driver/mysqli.php on line 255: mysqli_fetch_assoc(): Couldn't fetch mysqli_result
[phpBB Debug] PHP Warning: in file [ROOT]/phpbb/db/driver/mysqli.php on line 317: mysqli_free_result(): Couldn't fetch mysqli_result
[phpBB Debug] PHP Warning: in file [ROOT]/phpbb/db/driver/mysqli.php on line 255: mysqli_fetch_assoc(): Couldn't fetch mysqli_result
[phpBB Debug] PHP Warning: in file [ROOT]/phpbb/db/driver/mysqli.php on line 317: mysqli_free_result(): Couldn't fetch mysqli_result
[phpBB Debug] PHP Warning: in file [ROOT]/phpbb/db/driver/mysqli.php on line 255: mysqli_fetch_assoc(): Couldn't fetch mysqli_result
[phpBB Debug] PHP Warning: in file [ROOT]/phpbb/db/driver/mysqli.php on line 317: mysqli_free_result(): Couldn't fetch mysqli_result

Решение:

Удалить папку production в каталоге cache

Чтобы не выдавало ошибку HTTP ERROR 500

Выдать права: for anyone checking this, give your cache folder 777 permission
CODE: SELECT ALL

chmod 777 public_html/cache/ -R