Yes, I had the problem too.
Two mistakes I made:

• the default value for the pollen might not be a default - so better include the line in your config
• Did it for you already: I went to https://opendata.dwd.de/climate_environment/health/alerts/s31fg.json and checked if your part region exists. It does, so this is fine.

Also one mistake many people make is not to fully install:

cd ~/MagicMirror/modules # adapt directory if you are using a different one
git clone https://github.com/carstendirks/MMM-DWD-Pollen.git
cd MMM-DWD-Pollen
npm install # this can take a while

The important part is the npm install which installs dependencies!

For support always post your config. Here is mine - you can try my config to see if the problem is caused by a wrong config:

			module: "MMM-DWD-Pollen",
			position: "top_right",
			header: "Pollenwarnung",
			classes: 'scheduler',
			config: {
				// I use a schedule to only display it from spring to autumn! 
                                // module_schedule: {from: '00 8 1 3 *', to: '00 8 15 10 *' },
				updateInterval: 5 * 60 * 60 * 1000, // every 5 hours
				DWD_region: 41, // Rhein-Westfälisches Tiefland
				icon: true, // Show icons or not
				pollenList: "Hasel,Erle,Esche,Birke,Graeser,Roggen,Beifuss,Ambrosia", // Which Pollen to display?
				showNullValue: false, //Show value or symbol if value is 0
			}
		},

You might want to set showNullValue to true and also check in the json-Link if there are readings available (which currently there are)