Here you have the fix.
layouts/default.php (original)
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34 $logo_image = $this->API->get('logo_image', '');
35 $logo_image = $this->API->URLbase() . $logo_image;
36 $logo_image_header = $this->API->get('logo_image_header', '');
37 $logo_image_header = $this->API->URLbase() . $logo_image_header;
layouts/default.php (fixed)
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34 $logo_image = $this->API->get('logo_image', '');
35 if ($logo_image != '') $logo_image = $this->API->URLbase() . $logo_image;
36 $logo_image_header = $this->API->get('logo_image_header', '');
37 if ($logo_imager_header != '') $logo_image_header = $this->API->URLbase() . $logo_image_header;
If you don't do this, these variables never will be empty and will contain at least the value of $this->API->URLbase().
Regards....Antonio