I tried many hours to find out the problem, but I gave up after a long session. However I did trace it back to the image editor. It seems that when it is loaded/initialized after about 9 images or so it will crash with no error message. It seems like a memory overflow of some sort. I thought it was the max_execution_time issue but it doesnt seem to be that. What bugs me is that even when I turn on the WG_DEBUG mode, no errors are displayed at all. I also tried to get some answers with the error log file (WP_DEBUG_LOG) enabled, that didnt work either.
This is really a pain in the arse as I have many pictures to add to my blog and at the moment I can only add 8 at a time.
Here is the manual trace of the problem (as far as I could get):
/class.add-from-server.php Line 389: $data = wp_generate_attachment_metadata( $id, $new_file ); // Enters here...
/wp-admin/includes/image.php Line 103: $editor = wp_get_image_editor( $file ); // Enters here
/wp-includes/media.php Line 1179: $loaded = $editor->load(); // Fails when it executes during the 10th image. no error message
Here are some screenshots...