Update: This issue has now been resolved. Thank you for your patience!

Over the past 24 hours, some Groups users have experienced intermittent errors while attempting to post messages to the web interface. The team is working hard to investigate & rectify this issue.  Email delivery to these groups is unaffected, so on groups that allow email participation, we suggest posting and receiving messages via email.

We apologize for the inconvenience and will write back as soon as we fix the problem.

The Google Groups Team