Stuck in between slides - Loading Circle never finishes loading

Hello,

I already searched and found similar discussions, but none answered my question.

I´m updating a Storyline course and imported some new slides from a Powerpoint. When I want to preview the scenes made from the new slides I end up being stuck with an indefinitely rotating loading circle after the first slide.
The old scene in the same project works just fine.

I made sure that the triggers are set up correctly, the same master slide is being used, everything is the exact same as the working slides, but for some reason the new ones are broken and I can´t figure out why.
I even tried copying the working scene and just exchanging the slide content, but the problem persists.

Luckily when published everything works just fine, but still there is something weird going on, and I´d love to get this fixed to be able to preview in a normal fashion.

Thank you for your help