I know that this may sound overwhelming simple as a possible fix, BUT I' too have experienced the same "Write1 Error, cannot write sequence" issue, and have discovered that if you turn OFF the proxy view mode (in the active Viewer panel), the Write node will render the sequential frames (..of course you'll also need to have the output filename include the numerical pad string (i.e. "%04d") -But you probably already knew that...
Hope that helps,