There's sometimes a problem with the client when trying to submit solutions. Refreshing the page usually solves the "unknown error" issue.
The "6000ms" error is 99% of the time due to an unoptimized algorithm. The Kata details hint at the algorithm needing to work with big numbers. Iterating through all the numbers won't work in the time constraint.

Good call on testing with the numbers in the description. It would be great if the "Unknown error" could be changed to something else, as bouncing between that and the "It took long than 6000ms" error implies an issue with the site. Nonetheless, thanks for the guidance :)

Try testing with numbers in the range mentioned in the description. Also, try testing with negative n's.

I don't know if this is an issue with the kata's setup, or if it's the site itself, but my solution keeps failing to execute. All my unit tests are passing, including fib(1000), and they take < 1s. The kata's unit tests, however, aren't even running anymore. They just keep failing with "Unknown error" or "Process was terminated. It took longer than 6000ms to complete".