Kontakt GUI Designer - custom slider images don't load properly. What am I missing?

[Professor Fate]

Hi folks,
I'm making a UI for a sample library. In the past I used to build my interfaces in KSP, this is the first time I'm using Creator Tools and the GUI Designer.
I have a bunch of sliders and switches that i've skinned with custom png animations, 6 frames for the switches, 128 frames for the sliders. All png's are vertical strips and have their proper txt files. The main script gets loaded from the Resources folder and loads the nckp from the performance_view folder.
Whenever I load the instrument into Kontakt all images are being displayed as static instead of animated. The slider images are displayed just as they are, with all their frames. If the height of the instrument would be big enough I would see all 128 of them. The switches only show the first frame, no matter if clicked or hovered.
Now, when I go to an empty script slot, declare a control, assign one of the images to it and hit "apply" it gets displayed properly and the animation works as supposed, even in the performance view. I do the same with the other images and everything looks and works as it should. I save the instrument and close it. I reload it: everything is fine. I quit Kontakt, reopen it and load the instrument: back to square one, all images are static, frames are all over the place. It's like groundhog day. After two days of shouting at my screen I'm slowly going crazy. Before I'll physically attack my computer, please help me!

Additional info: I'm on MacOS 14.7, I'm building on Kontakt 6 for backwards compability. Instrument and project files are on an external SSD. I've already recreated the nkr several times which unfortunately didn't help.