in the file ~/.kde4/share/config/kwin.notifyrc, there was an entry in Event/close which was telling KTTS to say out loud "Good bye" (so lame of me, isn't it). I think I may have set that up a long time ago, to test KTTS, and I didn't remember doing it... anyway, I just cleared the KTTS line and removed "KTTS" from the Action line, and the popup doesn't appear anymore.

the weird thing is that the KTTS setting still works if the notifyrc file says so, but there's no GUI option to turn if on/off. I'm sure I didn't do this by editing a text file; some previous KDE version must have allowed me doing it via System Setttings. now it doesn't.

thanks for your help!