Chat window opens, but no content in message

I’ve noticed for the last few weeks, when I receive a message via spark the chat window opens with the recipients name at the top, but there is no content. I have to ask the sender to resend what they originally did.

This happens regardless of whether I already have a chat window open, but once I have sent them a message we can do 2-way communication.

I’m using 2.6.3 on OSX 10.7.5 with Java 1.7

Any thoughts?