Why is there a need to use the online installer ?
If you have avast installed use the regular avast program update function.
Are you using a VPN ?
If there happens to be any interruption in connection then it is possible the update would fail.
Personally I always prefer to download the full off-line installation file, that way I can have a backup copy of the off-line installation file should I need it in the future. As at some point in the future win7 may not be supported so having backup copies might come in handy if you need to go back to an earlier version.