Impossible to run online tests for Scheme / Accumulate

Running tests for this specific exercise systematically leads to the error:

An error occurred

An error occurred while running your tests. This might mean that there was an issue in our infrastructure, or it might mean that you have something in your code that’s causing our systems to break.

Please check your code, and if nothing seems to be wrong, try running the tests again.

This error occurs even with the default code:

(define (accumulate f xs)

where the expected behaviour would be to have failed tests.

Running online tests for other exercises works fine.