Note: If I submit the post nevertheless, the images appear in the posted message - they are there. But the previous stops working and I could not find any workaround to let it display the images again while I’m still editing the draft.
Another issue that might be either related to or the effect of the same bug:
Sometimes, when I Cmd+V an image from the buffer and into the editor, it uploads the image, but shows a wrong image in the preview.
The following screenshot demonstrateS how after pasting two different images into the text composer from the buffer with using Cmd+V, none of the images in the preview are what I have inserted: