If you're seeing the following error message "Topic overload! Consider splitting up your mind map into smaller maps. See here how.", it means your mind map exceeded 500 topics.
You can still create topics on such mind maps, however, you have to keep in mind that if you continue adding topics we can't guarantee that all features will keep working for this mind map.
Tip: There is a secret way to find out the number of topics on your map. You just have to click the word "Location" in your map properties. See number 3 on this page: Map Properties