All the writer nodes seem to not have write permissions on mapped network drives (on Windows 7 with domain integration, user names provided via the network).
A workaround is to use the actual server address (e.g. \\server\share) as suggested in this post:
But with auto-mounted network drives, the user often doesn't even know the actual address because it's hidden, so they simply have the impression that writing to e.g. M:\, N:\ etc. doesn't work.
I tried with CSV Writer, XLS Writer and Image Writer in KNIME 3.1.2.