BOB, your comment surely is useful to a lamer. Thanks!
EDDY, you put me down the right way. After the due mumbo-yumbo I got the time of disconnection down to twenty minutes; then thought some more and unchecked 'Loading after other programs' - and bingo! got Avast to behave! So I got the answer after all; and why 11th version behaved different is now perhaps a moot question.