The mod has stopped being updated again.
And this link no longer works.
The thing is that the HD mod has three update servers, and the error occurs because one of them (Dropbox) is unavailable. When an update is released, everything will download normally from the other servers, and Dropbox will be excluded from their list, and the error will disappear. Until then, you can interpret the error as "you already have the latest version."
Source of information: DrSlash, quoting Baratortcha on the DF2 forum.
