Derivative Images Plugin does not create derivative images

I have trouble with the Derivative Images Plugin: It does not make the derivative images, yet, I have no error alert.
(I am talking .jpg an .tif).

I get the normal message on the control panel:
“Processing files. This may take a while. You may continue administering your site.”

I get a normal data in the processes table (omeka_processes for me)
It first shows a PID, then status completed:
id class user_id pid status args started stopped
53 Omeka_Job_Process_Wrapper 1 \N completed a:1:{s:3:“job”;s:228:"{“className”:“DerivativeImagesJob”,“options”:{“process_type”:“all”,“derivative_types”:[“fullsize”,“thumbnail”,“square_thumbnail”],“mime_types”:[“image/jpeg”,“image/tiff”]},“createdAt”:“2016-12-20T12:00:40-03:00”,“createdBy”:1}";} 2016-12-20 12:00:40 2016-12-20 12:00:53

And I don’t get error messages in /var/log/apache2/error.log

But new derivative images in /files/* are not getting there.
Not even for files that have been manually deleted first to test this.
As far as I can see, write permissions in the /files/ directories are OK, which makes sense since I do get initial derivative images when uploading new files (that is, without any plugin involved).

This is with the following versions:
Ubuntu 16.04.1
Omeka 2.2.2
PHP 7.0.8-0ubuntu0.16.04.3
MYSQL 5.7.16
Derivative Images 2.0

NOTE, though, that I do get an error message in /var/log/apache2/error.log under other circumstances, like first access to the site, or searching. But with no other consencuences that I could see. I don’t know if this is related, since the use of Derivative Images Plugin is not followed by error messages.

PHP Warning:  session_write_close(): Failed to write session data (user). Please verify that the current setting of session.save_path is correct (/var/lib/php/sessions) in /var/www/omeka/application/libraries/Zend/Session.php on line 732

Any lead about what I should/could check?
Any help and suggestion appreciated.