The following string from language\en\acp*\group.php*
contains double space after "private message.":
'GROUP_MAX_RECIPIENTS_EXPLAIN' => 'The maximum number of allowed recipients in a private message. The maximum for all groups of the user is used to determine the actual value.<br />Set this value to 0 to overwrite the setting for all users of this group with the board-wide setting.',
I suggest removing one space so there is only one.