I have created a folder "Spam" in the roundcube webmail and selected the newly created folder "Spam" under "Special Folders" for Junk.
Now under "Folders" the folder Spam is displayed with the icon for the Junk folder as desired and the old folder Junk is now a normal folder.
If I delete the Junk folder now, I get the confirmation "Folder successfully deleted" but immediately after deleting it is there again (visible after reloading the page).