Since a couple of week, both dolphin and konqueror (using kio-ftp) are unable to copy/paste files from-to a remote FTP server. The case is very specific:

1) select a file on a remote FTP directory
2) copy it
3) paste on the same directory
4) you're asked if you want to overwrite or rename to a new file
5) choose to rename it
6) it says that it is impossible to create the new file.

I'm using trunk, compiled almost daily, the problem appeared the first time about a couple of weeks ago.