TYPO3 Core

• Create a new content element, e.g. a textmedia element.
• Go to the list module of the page, on which the content element was just created.
• Select mulit-selection mode for clipboard (if it has not already been selected).
• Go back to the page module.
• Copy the content element by clicking its top-left icon and selecting "Copy" from the context menu.
• Insert the copied element below/above the original element with the now visible "Paste after this record"-Buttons.

Site will reload, no error, but no copy has been created!

Also no record has been added to the tt_content table.

The array $this->clipData['tab_1']['el'] in the \TYPO3\CMS\Backend\Clipboard\Clipboard class is simply empty.
When switching to single-mode, $this->clipData['normal']['el'] will hold the correct tt_content reference.