I think there is a bug.
Passwords can not be changed.
New sites can not be created.
Existing entries can not be deleted.
Perform a repair of avast and check if the problem is solved.
If not, perform a clean installation of avast and check if the problem is solved.
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