Thanks for the hint. By the way, after adding a link you can not remove it again. Even if you remove it from the runtime.cfg the link will be still available. I made some research and found the new location of the link. After adding the first link a new file file will be created where the links will be stored.
The file can be found here: userdata\config\org\openhab\core\ui\tiles.config
Edit: After some more testing I can confirm that the path i posted there can be used for removing the link again. So you need to remove it in the runtime.cfg and the tiles.config. But if you want to modify the link this has to be done in runtime.cfg. A change in tiles.config is not updating the webui config. I’m not sure if this is the expected behavior but at least it works 