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It seems to work now, when I upload a new PDF. Thanks!

It seems that IMagick PHP Extension isn't installed. I set the mode to "Direct exec" and I now get the following. Not sure what path to set.

Debugging is on (1580066952)
Request options:
Array
(
    [skipNotification] => 1
    [caching] => 
    [debug] => 1
)
In use options:
Array
(
    [skipNotification] => 1
    [caching] => 
    [debug] => 1
    [width] => 400
    [height] => 400
    [geometry] => contain
    [pageNo] => 0
    [forceExt] => pdf
)
Processing: /var/www/vhosts/physicslab.gr/httpdocs/library/uploads/HM507.pdf
As: pdf
File size: 1797003 bytes (1.7 MB)
Creating lock: /var/www/vhosts/physicslab.gr/httpdocs/library/uploads/.filerun.thumbnails/HM507.pdf/1797003/400_400_contain[1].png.lock
Resizing using ImageMagick
Running: /usr/bin/convert -size '400x400' "/var/www/vhosts/physicslab.gr/httpdocs/library/uploads/HM507.pdf[0]" -verbose -auto-orient -flatten -thumbnail '400x400>' -strip -quality 95 "/var/www/vhosts/physicslab.gr/httpdocs/library/uploads/.filerun.thumbnails/HM507.pdf/1797003/400_400_contain[1].png" 2>&1
Resized image saved: /var/www/vhosts/physicslab.gr/httpdocs/library/uploads/.filerun.thumbnails/HM507.pdf/1797003/400_400_contain[1].png
Removing lock: /var/www/vhosts/physicslab.gr/httpdocs/library/uploads/.filerun.thumbnails/HM507.pdf/1797003/400_400_contain[1].png.lock
Caching is off. Removing resized image.