Event ID: 6398

I used to get the following error in event log:

The Execute method of job definition Microsoft.SharePoint.Search.Administration.SPSearchJobDefinition (ID 80d7c9cb-ece6-4631-9d61-5fa1a0e8596c) threw an exception

To resolve this, I did the following simple steps:

1. Go to Run command in SharePoint Server
   
2. Type Services.msc and enter
3. Locate "Windows SharePoint Services Search"
4. Right-click on it and select "Stop"
5. Right-click on it and select "Properties"
6. Navigate to Log On
7. Ensure it is running under domain account. Type the correct password
8. Click OK
9. In Services window, right-click on "Windows SharePoint Services Search" and select "Start"

Now, the above error message is gone!

Source: eggheadcafe.com