when entering group preferences, in the upper side of my screen some errors appear

[phpBB Debug] PHP Warning: in file [ROOT]/includes/acp/acp_groups.php on line 684: Invalid argument supplied for foreach()
[phpBB Debug] PHP Warning: in file [ROOT]/includes/functions_acp.php on line 137: Cannot modify header information - headers already sent by (output started at [ROOT]/includes/functions.php:3291)
[phpBB Debug] PHP Warning: in file [ROOT]/includes/functions_acp.php on line 137: Cannot modify header information - headers already sent by (output started at [ROOT]/includes/functions.php:3291)
[phpBB Debug] PHP Warning: in file [ROOT]/includes/functions_acp.php on line 137: Cannot modify header information - headers already sent by (output started at [ROOT]/includes/functions.php:3291)

if I do any chage and hit save, then this message appears

[phpBB Debug] PHP Warning: in file [ROOT]/includes/acp/acp_groups.php on line 406: in_array() expects parameter 2 to be array, boolean given**

changes are saved anyway

even though I have to say that I am trying to configure groups to receive group PM and I am unable to achieve that, is it possible that this issue is produced by the error?