I have the same error in here.
Craft info says i have installed the ImageMagick extension successfully and even use
‘imageDriver’ => ‘imagick’ in config file. But still cant upload GIF files.
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Issue is resolved, as @bradbell said the version for ImageMagick module and library was old and need to be update. You can check the latest relase from here: https://www.imagemagick.org/script/changelog.php
From some servers apt-get update won't really update the package. So you need yo install manually. There is a great how to article regarding this