Thank you for the update and additional information.
at first it was helpful, the missing picture is shown in the gallery after deleting the browser cache ..
This certainly suggests that the problem has been a browser cache issue. (As you are aware, clearing your browser's cache does not change anything on the web server and, if clearing your browser cache has resulted in you being able to see what you expect to see, then the files must have always been present and correct on the web server.)
it happens only - and everytime .. in galleries of the first gallery index page
Weird, indeed...
Here are a couple of things you could try to prevent browsers from caching your Showkase web pages.
Suggestion #1:
If you want to prevent browsers from caching Juicebox gallery XML files, then try defining the gallery XML files with a unique identifier (e.g. the current time which will be different each time the browser loads the page) via the configUrl option in each gallery's embedding code.
Open your 'showkase/_themes/base/pagetypes/juicebox.tpl' file in a plain text editor and change line 26 from:
baseUrl : '{$ss_pageUrl}/'
... to:
baseUrl : '{$ss_pageUrl}/', configUrl: 'config.xml?nocache=' + new Date().getTime()
Now republish your site for this change to propagate throughout all your Showkase Juicebox galleries.
Suggestion #2:
You may also be able to dissuade browsers from caching HTML pages by using certain <meta> tags.
Open your 'showkase/_themes/base/pagetypes/basetheme.tpl' file in a plain text editor and add the following code in the {block "meta"} ... {/block} section, between lines 9 and 23 (but not inside {if $ss_ogImageUrl ne ''} ... {/if}).
<meta http-equiv="cache-control" content="no-cache" />
<meta http-equiv="expires" content="0" />
<meta http-equiv="pragma" content="no-cache" />
Again, republish your site for this code to appear in all your Showkase pages.
if you give me your email...
If you ever need to contact me via email my email address is below my username here in the forum (visible only when you are logged in).
I hope my notes above help.