I don’t know what might be affecting everyone in particular here, but I had a similar problem with ruby/rspec. The problem was the command to run the tests was trying to output to a file. Normally, the tests would generate some kind of XML to a file for circle to read.
However, my tests were not just generating XML, but the ascii output that the test created, things like progress indicators, color codes, and meta information. Anyway, I fixed this for rspec by removing
--format progress from my test command. Obviously this will be different for each testing program. I found this by running the same command that circle is running and inspecting the output. What you want is pure XML with nothing else around it.