I tested this model and my conclusion is that the model is too thin in places where holes do appear.
There are at least three solutions that come to my mind at this moment:
1. Modify model to make walls thicker - requires some effort so I wouldn't recommed it if you don't want to waste time in 3d modeling software.
2. Rescale model so all walls are thicker - that solves problem of holes but makes another problem - you may not want such a big boots :)
3. Use offset function in "Advanced Print Settings" tab to offset outer contour - simpliest solution, this one I would recommend. I guess it's not a big loss in such kind of model to extend it a bit (different with models where dimensions do matter).
We are aware of this issue and don't have solution for it yet.