I realise this is a very old thread, but I have just encountered this problem with ZDesktop 2.1 in Windows, and this was the only thread that came up on a search.
I was able to resolve it by doing the following.
1. Ensure Z Desktop is closed (in windows, right click task-bar icon and choose exit).
2. Find your zimbra prefs file, on a windows 7 machine it is located here:
3. Add the following line to the bottom of the config
4. Start Zimbra & attachments should now open in read/write mode.
Not yet sure if this is a permanent solution to the issue though.