I keep getting the "a security token was missing" message whenever I try to access the User CP.

Do you know what style are you using?

Additionally try clearing your cookies.

...Sign out and sign back in, perhaps?

I changed the forum skin I was using by accessing the options after clicking on "Your Notifications." It was the forum skin I was using before that had that problem. If you're curious, the forum skin that had that problem was "Bottom of the Sea." I am able to access the User CP on the skin I've changed to.