This problem just started happening to me yesterday. I thought it was connected to the outages that were happening, but now not so sure.
When I try to sign in, I fail with an error code 8015190E. I have tried rolling back updates, clearing cache and reconfiguring my network settings (i.e., restore to default and resetting up my connection to my network).
So when I try to "Test Xbox Live Connection" I will sometimes fail to connect to the Internet, and sometimes fail to connect to Xbox Live. So I will run the test back to back numerous times and get different results without making any changes.