F.e. includes/functions_privmsgs.php

				// Additionally, do not include the sender if he is in the group he wants to send to. ;)

				if ($row['user_id'] === $user->data['user_id'])

				{

					continue;

				}

This code compares two strings to be exactly the same. If there is a MOD that casts the global user_id to int (which I think is a security addition), the comparison will fail.
(In this case it sends group pms also to the sender.)
The code should either check == only, or both values should be casted to int.