Try checking out the Windows Live Writer log file...
Unfortunately, mysterious errors happen; try reviewing the log file for clues. If your not sure, you can submit your log file to the Windows Live Writer Team for review; they might be able to tell you why the error is occuring.
Locating the Log file:
Open WLW >>HELP >>About WLW >>Show Log File
Collect that information and send that in an email to wlw-test@microsoft.com
Hope this helps.