Some tiles on the watercolour map are missing (you only see white)

[apuchau]

When navigating around this area: http://maps.stamen.com/watercolor/#17/51.40911/-0.36049 there are a lot of white spaces, and it's not a one-off connection failure, it's consistent. It seems to happen more in this zoom level, if I zoom out I can see those tiles (although there are still some tiles missing, but fewer of them)