This is a known issue that usually happens when resources are missing, even if the page itself is found and returns 200. It's reported here - the supremely horrible thing about it is that it doesn't always return 2 when something is 404. That's why I (and often I see others too) treat exitcode 2 and 0 the same.
I also started an issue to get the exit codes documented, but Antialize and the others don't really have time to update the project. (Although there are rumors of forking and 0.12, but they are not really substiantial yet.)
So, the first thing I would check is if there is anything that might cause a 404 (or anything else than 2xx actually) within the page. Something like a missing CSS, JavaScript or Image file.