TBP 3.04 problem? Cannot close "Account settings" dialog properly

A strange thing indeed: At one point the OK button at the bottom of the "Account Settings" dialog stopped working properly. I can still close the dialog by clicking Cancel, but OK doesn't work and hence no setting can be saved. Internet search turns up no solutions for me.
I tried a clean install to another drive, created a new mail account and the "Account settings" dialog closed as usual. I then copied my old Mail folder, as well as prefs.js and abook.mab, as stated elsewere in this forum. But I still cannot close the "Account settings" properly, so I suspect somtehing in my prefs.js file - any ideas anybody?