It's sometimes airline specific that you are trying to travel with, and usually occurs while making the final payment for the Zed fare ticket. I have noticed having a higher speed internet/wifi connection seems to mitigate this. Having said that, sometimes I have had to wait a while, called the said airline directly to list and even have someone else list me remotely from a different IP address to get around it.

I hope they get this figured out too, as it's annoying when you've got things to do to & places to be!