Ok, I fixed it myself: The server I wanted to connect to had a faulty CORS config. I only noticed that, because I used the developer console and saw the CORS error pop up when trying to connect to the WMS.
When disabling CORS on my browser (do not do that), I was able to connect to it and create the item.
Maybe the error message displayed by the portal could use some improvement to hint to what the response of the server is.