Andreas,
Can you go to the Media Library Folders’ Support page on your site and there click on the System Information tab. Information on the configuration of your site is displayed on this page. Please highlight, copy and paste this information into an email and send it to me at [email protected] and I see if I can reproduce this issue.
Thank you for the information. I can suggest three things that may help find the reason for this issue. First you can turn on WordPress debugging to see what warning, notices and errors are generated by your site.
To turn on WordPress debugging, edit the site’s wp-config.php file and change
define('WP_DEBUG', false);
to
define('WP_DEBUG', true);
define('WP_DEBUG_LOG', true);
define('WP_DEBUG_DISPLAY', false);
@ini_set('display_errors', 0);
(Be sure to change this back when you are done.)
This will allow different kinds of messages, some will not be related to Media Library Folders, to be written to a debug.log file in the wp-content folder. Now try uploading a file. If you find something in the debug.log related to this issue then please send the file to me at [email protected].
You can try deactivating all your other plugins and see if the issue continue to happen. Also when the file upload fails you can try opening Media Library Folders in a new tab, go directly to the destination folder and try uploading again.
Looking at your debug.log, it is unusual that the uploaded file is not found when the plugin tries to move it. Perhaps there is a permission or file ownership problem with saving the file on the server during upload. You should talk to your host provider about the issue.