Upgraded from 11 version to the 12.3.2280 and - voila! - the new version blocks Internet access for a hour. What do I need to remedy this irritation?
Perform a repair of avast and check if the problem is solved.
If not, perform a clean installation of the latest avast version and check again.
If still not solved, provide details.
What OS/SP ?
What exact version of avast ?
Is there any other security (related) software installed ? (or was there)
etc.
@Eddy,
Instruction for repair and clean install would be helpful. 
Repair Avast:
Control Panel> Program and Features (Add/remove program)>Select Avast> Select Repair. Reboot when completed
https://www.avast.com/faq.php?article=AVKB204
If Repair doesn’t fix the problem, try the following:
Clean Install of Avast:
https://goo.gl/4Ptzkf
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.
My pleasure glad to hear that everything is working again.
It seems to me you have (many?) things loading at boot-time that are needed to do so.
If you want we can have a look at it.
Get and run Farbar and attach two log files (FRST.txt and Addition.txt) to your next post.