Codeigniter Image Resize with Big Images Not Working - image

Question

I've created a codeigniter website with a image resize function. This website worked great on my old webspace, now I transfered the website successfully to my new webspace, but unfortunately only small file size images are resized.
Big size images are not resized, and there are no errors displayed.
I've already checked the following stuff:
memory_limit=64M, GD2 image library installed on server, upload_max_filesize=25M, post_max_size=25M, max_input_time=300, max_execution_time=300
Here's the resize code I use:
$query = $this->db->query('SELECT * FROM table1');
foreach ($query->result_array() as $key => $row)
{
//$key value on error is '0'
$config_1['image_library'] = 'imagemagick';
$config_1['source_image'] = $filePath . $fileName;
$config_1['new_image'] = $filePath . 'resized1' . $fileExtension;
$config_1['maintain_ratio'] = TRUE;
$config_1['width'] = 420;
$config_1['height'] = 680;
$config_1['quality'] = 100;
//+ 2 other times the same for $config_2 and $config_3
$this->load->library('image_lib');
$this->image_lib->initialize($config_1);
$this->image_lib->resize();
$this->image_lib->clear();
$this->image_lib->initialize($config_2);
$this->image_lib->resize();
$this->image_lib->clear();
//function quits here with big files (tested with 11 MB files)
$this->image_lib->initialize($config_3);
$this->image_lib->resize();
$this->image_lib->clear();
}
//Update: I've now tried to change image library to ImageMagick to see if it works. Now I get the following error (and images are not resized):
A PHP Error was encountered
Severity: Warning
Message: filesize(): stat failed for test-uploads/files/2016/02/01/resized-file.jpg
Filename: controllers/xyz.php
Line Number: xyz
The error occurs here:
$photo_size1 = filesize($filePath . $fileRename_1) / 1024;