I sometimes see the following error message when using the web based webmail 2 service. What does it mean?
Error in template 'mail.Message#MessageHeader': TypeError: c._msg is undefined
The work-around to this issue is to enable the Reading Pane in your preferences. After doing so, you may need to log out and back in.
If you see this error http://i.imgur.com/kQMaih.png
You enable the reading pane http://i.imgur.com/NcM43.png
and then you should see attachments http://i.imgur.com/nqTzKh.png
Have you made any recent changes to your setting?
If not restore them to default.
This should resolve your problem
That error can come up when you try to look at an email with a photo or attachment. It comes up when the reading pane is turned off. But as radioactivemanmentioned, turning on the reading pane should resolve the issue.
Another fix is to change the display option from Advanced (AJAX) to Standard (HTML):
Retrieving data ...