I tried a lot the last days using a virtual server with a new XWiki-Installation.
The result is:
When a subwiki is created all works fine.
After switching the default language to German, the “Home” page is gone and a new “Startseite” is created (the content of the Englisch “Home” content is completely gone and replaced by the default content of a new created subwiki home page. And exactly in this new automatically created “Startsteite” the problem occurs. And only there. All other pages work fine. Also new created pages work fine, only on the “Startseite” (:8080/xwiki/wiki/testwiki2/view/Main/?language=de#) drag and drop a images, are well shown, can be perfectly saved, but after reloading the page the attachment is not existing anymore and the link to the image on the page have become orphaned.
Creation of a new page and making this page to the new HOMEPAGE (Wiki Administration->Wikis->Descriptor->HOMEPAGE) “solves” the problem.