Thumbnails creation failing when upgrading to rc1

I am testing the rc1 version and I cannot generate thumbnails.
I upgraded from beta 4, and with beta 4 GdThumbnailer was working (there were already problems with ImageMagick and Imagick, with no apparent reason either).
What could be the cause ?

Here is the message in “application.log” after loading an image without thumbnail creation:

2017-10-24T09:34:16+00:00 ERR (3): Command “/usr/bin/convert ‘/tmp/omekaEcncZm[0]’ -background white +repage -alpha remove -thumbnail ‘800x800>’ ‘/tmp/omekahqHIi2.jpg’” failed with status code 1.
2017-10-24T09:36:10+00:00 ERR (3): Command “/usr/bin/convert ‘/tmp/omekacryeNs[0]’ -background white +repage -alpha remove -thumbnail ‘800x800>’ ‘/tmp/omekafmckdm.jpg’” failed with status code 1.

In case it can help…

Try to run the command manually on your original file and check if there is an error.

If you were using GdThumbnailer before, the issue is that the structure of the user configuration for thumbnailers has changed a little between beta4 and rc1.

The thumbnailer is now configured more similarly to other components of Omeka S, which causes some of the keys to move around. The current way to switch thumbnailers looks like this:

'service_manager' => [
    'aliases' => [
        'Omeka\File\Thumbnailer' => 'Omeka\File\Thumbnailer\Gd',
    ],
],

As for ImageMagick not working, the usual and likely cause is simply that your ImageMagick is too old. Version 6.7.5 or later is required (you can check yours with convert -version).

Thank you very much for your answers.
I made the change to Gd in module.config.php as you suggested and it works fine now.
I would like to be able to use ImageMagick at some point though and you were right: the version is too old on the server I use for the tests.