Thanks! You have bones with zero scale in setup mode. When the project loads, it tries to zoom the viewport camera so the skeleton fits. Something in the skeleton (I'll look later, it's midnight on Saturday 😛) is using the zero scale in computations, causing NaN (not a number) values in bone world transforms, which then break the camera.
I've fixed the camera so at least this problem won't make the app unusable. The skeleton will disappear though and the camera will zoom in on nothing.
To fix your project, set nonzero scale on bones under Iono - Po2. Unfortunately that's not enough to clear the bad data in bone world transforms, you also need to save and reload the project.
Generally it's not useful to set zero scale in setup mode. Scale from animations is multiplied by the setup scale, so if it's zero scale will always be zero, you won't be able to key it to other values. Spine prevents entering zero in setup mode, changing the value to 0.00001. I wonder how you get zeros? Maybe if it was done in an older project, before we started preventing it.
