A further thought:
The site experiencing the problem after updating to 0.7.7 is a WPML site in two languages. To solve the need to have business listings in both FR and EN, I duplicate a listing, change its info to the other language, and assign it to another category. Could it be this duplication that’s sending new headers?
Also: Is it safe for me to delete the plugin and do a fresh installation? Or will I lose all my data doing that?