I've tried a second time using a brand new Trailhead Playground, and still get the error when checking the challenge:
"Challenge Not yet complete... here's what's wrong: 
There was an unexpected error while verifying this challenge. Usually this is due to some pre-existing configuration or code in the challenge Org. We recommend using a new Developer Edition (DE) to check this challenge. If you're using a new DE and seeing this error, please post to the developer forums and reference error id: VRHJDGJD "

I am having trouble completing the challenge for the "Create and Edit Lightning Components" module of the "Lightning Components Basics" trail. Here is the challenge:

Create a camping component that contains a campingHeader and a campingList component.
    The campingList component contains an ordered list of camping supplies that include Bug Spray, Bear Repellant, and Goat Food.
    The campingHeader component contains an H1 heading style with a font size of 18 points and displays 'Camping List'.

Here is my code:

camping.cmp

<aura:component >   
    <c:campingHeader/>
    <c:campingList/>
</aura:component>

campingHeader.cmp

<aura:component >
    <H1>Camping List</H1>
</aura:component>

campingHeader.css

.THIS {
}

H1.THIS {
    font-size: 18px;
}

campingList.cmp

<aura:component >
    <aura:attribute name="campingSupplies" type="List" default="['Bear Repellant', 'Bug Spray', 'Goat Food']" />
</aura:component>

When I check the challenge, I get the error message:

Challenge Not yet complete... here's what's wrong:
The component is not using the correct font size.

Any help is greatly appreciated.