I have something similar when running the test cases in Python. Sometimes the console output gives the following output, regardless if the output provided by my code is correct or not:
Running the test case several times in a row will sometimes result in this strange message while most of the time it seems to behave correctly. I have had this with several of the puzzles, so I don’t think this is related to any specific puzzle. Most recently it was with the sum of spirals diagonals puzzle.
Screenshot:
