You shouldn’t have to what you have experienced is far from normal.

I really have no idea why it has happened (or if it were database corruption, that has to be an educated guess), if it were a regular occurrence/bug then there would be much more activity on the forums.

I have no idea what you mean by this “I keep getting 3S, 4S, etc. messages” What is 3S and 4S ?

Getting this invalid signature message means that when avast tries to validate the downloaded incremental virus database update it is failing validation. So something is messing with the actual download as it isn’t all being received and I have no idea why that is (poor internet connection). But I don’t have any way of testing that.