Each time I open Dolphin, I'm prompted whether I want to login with Nextcloud. I find this quite annoying and since I don't really use Nextcloud often, I want to remove Nextcloud from Dolphin so I don't have this prompt anymore.
I deleted the WebDav share under the Dolphin Network tab and my I also deleted the login credentials from the settings app.
But I'm still prompted:

You are about to log in to the site "cloud.ist-toll.xyz" with the username "Ben", but the website does not require authentication. This may be an attempt to trick you.
Is "cloud.ist-toll.xyz" the site you want to visit?

Does anyone know what to do to rid of it?

It's been now awhile and the nextcloud prompt suddenly went away. Maybe because of an update or something, I didn't do anything.
As much as I'm sorry for those who have the same problem, I have absolutely no idea how I fixed it.