I can help you directly with this. I can assure that this is a temporary setback, and you will soon be able to update normally like before.
Now, first please tell me what is your latest update, as in the picture in the attachment.
Now, manually update and tell me where the file stops downloading itself, it should be a .vpx file.
This is not a problem with your computer but a temporary problem with the server. I can manually give you the files and tell you where to put them in the avast folder, that way, when the server is running properly again, you can update as usual.
Also, this problem is very temporary, you should be unable to update only for a day. If more then you might have a problem with your computer but it is highly unlikely.