Cannot post reliably - keep getting errors

I am having great difficulty in posting anything that is not straight text – I suspect a WordPress bug (recently upgraded to 2.9) but I cannot narrow it down. It appears to be when I have links to external sites. Wonder if WordPress introduced a new security feature?
Got it – any post created with the twitter upload plugin active and WP 2.9 or greater gets garbled. Disable and delete the plugin, then create new posts – all is good.
Set up with different plugins and theme.