Thank you for your reply @horst.
Exactly, the module "Animated GIF Image Sizer" was shipped within the PW core.
This is how i installed it: In the backend i went to module > core. Then I scrolled to the module "Animated GIF Image Sizer", then clicked on the "Install" Button next to it. Then the module page opened and I clicked on the "Submit" Button in the upper right corner.
OK, I tried enabled the debug mode, then deleted the cached versions of the GIF, and reloaded the frontend in order to re-regenerate the resized GIF.
It's strange, because in setup > logs > image-sizer I am getting this message which seems to be fine:
AnimatedGif Resized: rocket.gif => rocket.453x575.gif (453x575) 0.1704 secs 154457 => 36848 bytes (quality=80, sharpening=soft)
I am not getting any error messages or something, but the resized GIF is still not animated.
Any idea, what it could be?