One more thing: it seems like saving a completly empty object causes the saving progress bar to get stuck indefinitely (I wanted to use one with Set Entity Model to replace objects which aren't supposed to be here anymore, but I could use an empty animation frame I guess).
EDIT: This seems to a pretty specific bug. I've tried a few things and clearing a model with the Clear button works fine, but deleting manually all voxels in a frame then saving triggers the bug. It also seems to happen no matter the number of frames in the model.