This happens to me everyday, wherein I have a new message from someone but then I can't open it. So, what I have to do is delete their messages and then make a new one and text them what they have wrote on their previous message to me. I can't do that always because my friend and I already have a long conversation but because I can't open his sms all of a sudden, I need to delete it and remove the past conversations we've had
This used to happen to me, when I was running .406 (I believe.) A soft reboot would usually fix it. The easiest work around I've found is to just hit menu, choose compose sms, and type in the same contact that you're locked out of the thread of. Instead of opening a new thread, it'd just open the one that I couldn't previously get into. A little inconvenient, but way nicer than wiping the conversation every time it happens.
Since I upgraded to .938, it hasn't been an issue to me though.