Why can't I update my virus definitions?

[REDACTED]

I am a user of avast! Internet Security. Recently a window which says "Your virus defeinitions are out of date. You should update them now." popped up several times from the orange avast! icon in the system tray of my computer. But every time my effort to update the virus definitions ended in failure and the software always told me that the host was unreachable and the engine and definitions were already up to date. Can anybody tell me how to fix this problem?

[TwinHeadedEagle]

Hello,

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Please download Farbar Recovery Scan Tool and save it to your desktop.

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[Asyn]

I am a user of avast! Internet Security. Recently a window which says "Your virus defeinitions are out of date. You should update them now." popped up several times from the orange avast! icon in the system tray of my computer. But every time my effort to update the virus definitions ended in failure and the software always told me that the host was unreachable and the engine and definitions were already up to date. Can anybody tell me how to fix this problem?

Repair Avast:
1. Control Panel -> Add/Remove programs -> Avast
2. Click on 'Repair'.
3. Follow instructions.
4. Reboot.

[REDACTED]

I have a very similar problem.  I am also getting a message that says "Your virus definitions are out of date. You should update them now". But on the overview tab, it says that everything is up to date.



And when I try to update the definitions manually it says "Unknown error".





This problem started to occur after I installed "anti-WannaCry" updates for Windows (KB4012212, KB4012215). This might be just a coincidence, but I reckon I should mention this.
I tried to repair Avast, as Asyn suggested, but with no luck.

[Eddy]

Change your DNS to Googles DNS and try again.

[REDACTED]

I don't know what exactly has changed, but today my Avast successfully automatically updated its virus definitions. This happened before I even touched any settings, like DNS etc. This is really weird.

[mchain]

You can put the issue down to an unknown issue with an avast server.  You were not able to connect to it.

Problem was not with your machine, at least in this case.

Happens.

[REDACTED]

It happens again. 
Moreover, it happens on all my PCs. I have 2 desktops and 1 laptop with Avast 2015 under Windows 7. I know that it's an older version, but I prefer the UI of this version over the newer ones and would like to continue using it. AFAIK they discontinued virus definitions updates only for version 4.8, which was released in 2009.
I tried to use Googles DNS, but it didn't help.
I also tried to connect avast to the internet using a proxy connection, but the outcome was exactly the same:

[Eddy]

https://www.avast.com/download-update

Why are you still using a old version (2015) ?
You are missing several protection layers.