In Dolphin, contextual menu of the folder to copy, and click in "Copy". Then, at destination folder, click in "Paste One Folder".

Occasionally? for no reason, I can't copy folders.

Similar to Opensuse bug : 
Most of the time, when I try to copy a folder (with files inside), I get an information message saying: "/home/user/folder1 is a folder, but a file is expected.", and the folder is not copied.

I cannot copy, you have to cancel.

Work around. Opening a new instance of Dolphin and you can then copy.

Reproducible: Sometimes

Steps to Reproduce:
1. In an open Dolphin window.
2. In Dolphin, contextual menu of the folder to copy, and click in "Copy".
3. Then, at destination folder, click in "Paste One Folder".


Actual Results:  
Most of the time, when I try to copy a folder (with files inside), I get an information message saying: "/home/user/folder1 is a folder, but a file is expected.", and the folder is not copied.

Expected Results:  
Move or copy the folder

https://forums.opensuse.org/showthread.php/511765-Dolphin-can-t-copy-or-move-folders

Kubuntu 16.04

I have put it as "critical" as this is a basic required operation, could fail to copy data (by not coping at all), or (others have reported) possibly may not fully move Folders recursively.