Often, when I'm browsing an SFTP (SSH) network connection, doing any kind of operation (refreshing the view, pasting files, etc.) will simply result in an "Internal error". No further information given. Going back to the "Network" sidebar entry and then navigating to the connection and the whole path that was open will often fix it.
I think this bug (and all the other similar dolphin bugs I just reported) might be connected to the "Folder" field (when creating the connection through the wizard) not being "/". When I use "public_html" as the folder, all the errors occur. If I use "/", it seems to be more stable.
I see the same issue, but only if I try to copy from one SFTP folder to another using the split-pane view. I'd love to provide more detail, but I am unable to find any KDE or Dolphin log file.
*** This bug has been marked as a duplicate of bug 413294 ***