Ok, i have fixed it for now by adding a new variable with an if statement to the meta-box.class.php file in line 100:

$custom_priority = isset($box['priority']) ? $box['priority'] : 'high';
add_meta_box( 	
	$box['id'], 		                                // HTML 'id' attribute of the edit screen section 
	$box['title'],		                                // Title of the edit screen section, visible to user 
	array(&$this, 'create_meta_box'),		// Function that prints out the HTML for the edit screen section. 
	$area, 							// The type of Write screen on which to show the edit screen section ('post', 'page', etc) 
	$box['context'], 					// The part were box is shown: ('normal', 'advanced', or 'side').				
	$custom_priority,					// The priority within the context where the boxes should show ('high' or 'low')
	array('avia_current_box'=>$box) 		// callback arguments so we know which box we are in
); 

But i still don’t understand why this issue occurred at the first place? Why do some of the boxes not have the priority set?