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Zero Are Child Patch
Description:
A solid in your level has a face with no area. My guess is that vrad.exe is trying to create a lightmap for a face that doesn't exist. Maybe you have an invalid brush?
Solution:
First check Hammers problemchecker (alt+p). If that doesn't solve your problems, you can either ignore the error, or try to find it anyway using cordon tools. You would be looking for brushes which might be too complex. I wouldn't recommend the second option.