I have the same issue as the org, poster. I found out this happens when i have alot of SMS's on my phone. Like ALOT (i average 5k-7k per month)

The only way to fix this is by apparently Deleting the messages from everyone , if you try this it will work 'normally' again untill you fill up your texts to that certin amount and once again you would have to delete again.

Like i said this is the only way it works for me, if someone has a permanent sollution without the need to delete the txts let me know as i love to keep mine without deleting too

*note* on my n900 i never experienced this problem and i had alot of texts there too!