I've seen it joked about a few times by Swedes and Norwegians that Danish is so difficult to understand that not even Danes understand it and occasionally have to switch to other Nordic languages to have a conversation.
I thought it was just a joke, but after traveling a lot to Norway and Sweden I've realized that surprisingly many Norwegians and Swedes honestly believe Danes can understand Swedish/Norwegian even when they (the Swedes and Norwegians) don't understand Danish.
I'm sorry to disappoint, but the trouble of understanding goes both ways. Of course it can be done depending on where you're from, how slow you speak, and how willing you are to listen, but I know a few people who work in tourist areas and they all say they understand the Germans better than people from other Nordic countries, even if they didn't have German in school.
So for the joke to be more correct it should be that Danes understand no one. Not Swedes, not Norwegians, not Danes, but a bit German.