Error 0x80240438 followed by a message stating “virus and spyware definitions couldn’t be updated” is a Windows Defender error usually seen in Windows 8 and 10. The error states that updates have been interrupted. There are two possible reasons for this to occur, the first one is that the windows update servers from where the app is receiving updates is down or is having issues which is quiet common with Microsoft, and the second one is that your router or firewall has/is blocking the connection to the update server.
Troubleshooting your Firewall to fix Error 0x80240438
The first approach i always take is to disable the Firewall entirely. This helps me check if it is indeed a firewall issue before doing anything else and in most cases disabling it fixes the issue.
You can easily disable third party firewalls by pointing your mouse to the av/fw icon on the bottom right and doing a right click. When you do that; you will get the options to choose from; use the one that says disable. These are generic steps that applies to most of the anti-virus software’s with firewall.
If this fixes your issue; then re-enable firewall without doing anything else. Otherwise; proceed to the new method below.
Reboot your router
Sometimes your router can also block the connection to the update server. I always recommend doing a power-cycle. You can do this by turning the router off for 2-3 minutes and then power it back on.
Manually download the update definitions
Microsoft has a way to download updates manually. Since everything is described here. You can easily follow the steps by visiting the link.