Why is my legend empty or incomplete ?

Whenever I am printing a map including one particular Map Service, the legend is either missing or incomplete. (Geocortex Printing 5.9)

Would you know what could be the reason, and how to solve the problem ?

I looked in the Chrome console and network logs, but I couldn't find any weird behavior.

? @Paul Van Haaren?  @Ken Lyon? or @Darren Boss?  maybe ?