When I try to join my friends' realm, it gets stuck on "Building Terrain" and doesn't proceed.

I log in, go to Friends, wait for the realms to pop up and click on my friends' realm. a purple screen (like a nether portal) comes up and says: Joining realm, loading resource packs etc. However it gets stuck on "Building Terrain" and just gets stuck on that however the loading bar is still moving. all other realms I try to join work succesfully. asked my friend if he changed anything about his privacy setting etc but he changed nothing. kind regards, Seth