I really do not know how long it will be until the next version of Juicebox-Pro is released.
It is unlikely that a new version will be pushed out quickly to fix a single bug but I have logged a bug report with the developers and the issue will certainly be investigated and should hopefully be fixed in the next development cycle.
There is no set release schedule and, unfortunately, any estimate I give you (weeks vs months) could turn out to be wholly inaccurate. I hope you understand.
However, if you'd like to be notified when new versions are released, you can join our mailing list at the foot of our homepage, keep an eye on our Facebook page, follow us on Twitter @JuiceboxGallery or subscribe to our blog RSS feed.

Once the bug has been fixed, all you need to do is swap your gallery's current 'jbcore' folder with the new 'jbcore' folder. You can do this manually or by opening and re-saving your gallery in the new version of JuiceboxBuilder-Pro (when it is released).
However, as the bug-fix could be a while away, I would advise using a different set of configuration options (set buttonBarPosition to OVERLAY or TOP and/or set thumbsPosition to TOP, BOTTOM or LEFT) until the next version is released.

I think the space you are referring to is due to the fact that you load your gallery into a iframe.
When loading a gallery into an iframe, the Splash Page is not used (as the gallery would be expanded only within the iframe) and the thumbnail page is initially displayed instead.
Your iframe has been assigned a height of 720px and there are only 5 thumbnails to be displayed in this area which results in space above and below them.
Possible solutions would be to either reduce the height of your iframe or, better still, embed your gallery directly into your web page instead of loading it into an iframe.
As you have already uploaded your entire gallery folder to your web server, you can use the baseUrl method of embedding documented here.
The baseUrl method of embedding is essentially the same as the regular method of embedding (documented here) but with a baseUrl entry in the embedding code pointing towards the gallery folder.

Try replacing your <iframe> code with the following embedding code:

<!--START JUICEBOX EMBED-->
<script src="/Gaestegalerien/test/jbcore/juicebox.js"></script>
<script>
    new juicebox({
        baseUrl: "/Gaestegalerien/test/",
        containerId: "juicebox-container",
        galleryWidth: "100%",
        galleryHeight: "720",
        backgroundColor: "rgba(34,34,34,0)"
    });
</script>
<div id="juicebox-container"></div>
<!--END JUICEBOX EMBED-->

Once the gallery has been embedded directly into your web page, the Splash Page will initially be displayed in your web page (when viewed on mobile devices) and, by default, the first image in the gallery will be used to fill the Splash Page (so there will be no blank space within the gallery area). When the Splash Page is clicked/tapped, the gallery will be expanded to fill the browser viewport where the images will have more room to be displayed. (The user can return to the embedding page via the 'Close Gallery' button on the Button Bar.)
You can configure the Splash Page (for example to change the Splash Page image) using the Splash Page configuration options.

More information about the Splash Page can be found in the Screen Modes section of the Gallery Tour.

I hope this helps.