ACP > Manage Users > Trace Permissions (little arrow)

For a regular user (non-admin) on any of the trace permissions.

[phpBB Debug] PHP Notice: in file /includes/acp/acp_permissions.php on line 951: Undefined index: username
[phpBB Debug] PHP Notice: in file /includes/acp/acp_permissions.php on line 1049: Undefined index: username
[phpBB Debug] PHP Notice: in file /adm/index.php on line 136: Cannot modify header information - headers already sent by (output started at /includes/functions.php:2110)
[phpBB Debug] PHP Notice: in file /adm/index.php on line 137: Cannot modify header information - headers already sent by (output started at /includes/functions.php:2110)
[phpBB Debug] PHP Notice: in file /adm/index.php on line 138: Cannot modify header information - headers already sent by (output started at /includes/functions.php:2110)

Also got this error on another user for any of the Administrative Permissions set to "Yes":

[phpBB Debug] PHP Notice: in file /includes/acp/acp_permissions.php on line 1101: Undefined variable: user_row
[phpBB Debug] PHP Notice: in file /adm/index.php on line 136: Cannot modify header information - headers already sent by (output started at /includes/functions.php:2110)
[phpBB Debug] PHP Notice: in file /adm/index.php on line 137: Cannot modify header information - headers already sent by (output started at /includes/functions.php:2110)
[phpBB Debug] PHP Notice: in file /adm/index.php on line 138: Cannot modify header information - headers already sent by (output started at /includes/functions.php:2110)