error when sending message in Curriki Leaders group

1. Went to Messages tab for Curriki Leaders group.
2. Clicked Create Message.
3. Entered subject, message content, and selected sending to whole group plus email notification.
4. Sent message.

Received this screen:

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Proxy Error

The proxy server received an invalid response from an upstream server.
The proxy server could not handle the request POST /xwiki/bin/view/Group_CurrikiLeadersGroup/CreateMessage.

Reason: Error reading from remote server
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Thought had not sent message, so resent. Received two message notifications in my email, one at 12:57 and one at 1:04. Both work on clickthrough. Both appear in listing.

This has happened before.