After updating phpBB to v3.0.4, this error suddenly popped up after changing the language to something other than English :
[phpBB Debug] PHP Notice: in file /includes/functions.php on line 3526: Undefined index: HIDDEN_USERS_ZERO_TOTAL_AND
It means that the language pack is outdated and “HIDDEN_USERS_ZERO_TOTAL_AND” isn’t defined in the common.php file in the corresponding language directory.
The solution is to install the updated language pack for the current version from the language pack directory, or if the language hasn’t been updated yet, wait for the maintainer to update it; or better still, update the language pack yourself!