I see the problem in your gallery occasionally (but not always) in Chrome 56 and IE11 (but I've not seen the problem in Edge, Firefox 51.0.1 or Opera 42).
However, I have been unable to reproduce the problem in a test gallery of my own (using the same configuration options that your gallery uses and using your custom theme).
It looks like it might be a difficult problem to troubleshoot.
It looks like it might be some kind of timing problem. Juicebox might be positioning the Button Bar at the top of the page before knowing that the thumbnails are there. If this is the case, then it's certainly strange that I cannot reproduce the problem myself.
Try taking the gallery out of your web page and make it a standalone gallery (with no other content on its page) to see if this makes a difference and to see if the code on your web page is somehow contributing to your problem.
Also, it would be easier to troubleshoot a standalone gallery (with no external influences) and figure out which combination of configuration options is causing the problem (if, indeed, this is the cause).
Try also using the stock 'theme.css' file just in case any of your custom CSS is interfering. It doesn't look like it is but the more we can strip back, the more we can eliminate as being a possible cause.
I do not see any custom CSS on your web page which should be interfering with your gallery's Button Bar position but I might be missing something so have a look and see if you can find any custom CSS which the gallery might be inheriting.
Being that it looks to me like a timing problem, you might also like to try changing internal timings (such as displayTime and imageTransitionTime) to see if this helps.
Also, for testing purposes, try adding a delay before the loading of the gallery when the 'Foto's' link is clicked.
I'm not sure what this will tell us but it's something I would try out of interest.
I realise that the suggestions above might not be ultimate solutions but they might at least help to track down the cause of the problem and, if it turns out to be a bug in Juicebox, I can at then log a bug report with the developers.
Thank you.