I replied to your query above via email on 17 March 2017.
I hope you received it (but being that you are posting your query here, I'm guessing that you might not have).

Below is a copy of my message. I hope it helps.

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I have viewed your web page in Chrome, Edge, Firefox, IE11 and Opera on a PC and see similar results as yourself. Both galleries display fine in all browsers except Firefox (where only the second gallery is displayed).

I notice that the first gallery displays fine in Firefox when viewed on its own web page here: http://www.sakura-karate.de/juicebox/07 … index.html
... so the problem seems to be somewhere within your embedding page.

It looks like the problem is caused by some custom CSS in your 'sakura.css' file.
The first Juicebox container on your page (relating to the first gallery) has a CSS class of 'kurznews':

<div class="kurznews" id="juicebox-container02"> 

... and your 'sakura.css' file has the following code on line 1329:

.kurznews {
    display: table; 
    width: 100%; 
    border-bottom: 1px solid #888;
    margin-bottom: 30px;
    padding-bottom: 40px;
    float: none;
    clear: both;
}

All this custom CSS (including 'display: table;' which I believe to be the cause of the problem) is being applied to your first gallery's Juicebox container.
The second Juicebox container on your page does not have this CSS class (and displays fine).

Remove the 'kurznews' class from your first Juicebox container (or at the very least make sure that 'display: table;' is not applied to the Juicebox container) and the gallery should hopefully display fine.

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Try changing:

<div class="kurznews" id="juicebox-container02"> 

... to:

<div id="juicebox-container02">