When I click on the Article button at the bottom of the editor, then none of my items are showing, just a window with an empty list. Why are not my items showing in the list?

you will need jce and this
getk2.org/extend/extensions/date/item/869-k2-links-for-jce

I installed and enabled these plugins. Still the Article button at the bottom of the editor shows a empty list. Also the "Insert/Edit link" button in the toolbar doesn't find any of my items (articles), only the menu items.

I found the items when manually browsing the editor window under "K2 Categories & Items"->Category name->Item name. The search function did not find it.

Having same issues here. Installing the JCE-plugin solved the problem with no K2-articles showing when adding links.

But I still have the issue that the article-button in the editor footer only shows the "old" Joomla-articles and none of the K2 articles. Is there any solution to this?