PHP Warning: SimpleXMLElement::addAttribute(): Attribute already exists in C:\Users\Admin\Documents\Development\Git\carddav2fb\src\FritzBox\Restorer.php on line 150
Okay, new day, new start - could reproduce what I saw once before: it´s just a Warning.
But to avoid it I found the way (and understood why the RestorerTest failed)
Warning occures in background image upload: