I'm also having the same problem. I thought it was an issue that suddenly appeared after updating the plugin to the recent version (version 1.4.2).
Reverting to the old version of the plugin didn't help. I still couldn't edit the SEO title.
However unlike the last two comments above me, I don't get the error message (The plugin generated nnn characters of unexpected output during activation. If you notice “headers already sent” messages, problems with syndication feeds or other issues, try deactivating or removing this plugin.).
It's simply that I couldn't edit the SEO title. It's greyed out. That's all.
UPDATE: I've reverted to version 1.3.4.4 and the SEO title can be edited once again.