For more than a week, when I try post a message in the Google Reader Group, I get this error message: "An error was encountered while trying to post, please try again later." It only happens for a single Google Account and only for Google Reader Group.

I've noticied that error too, few months ago on a private google group. It happend when I posted a message on a thread by email, then I posted another message on the same thread, via the web interface.
But hours later it was gone.