Had a similar problem on Windows. Got an error that the Download Assistant was not able to connect to the server. I downloaded and installed the update of Dorico directly from the website: Dorico 5 Updates and Downloads | Steinberg
Nothing that will be of any help to you, I’m afraid. They have had a couple of other reports of this problem, but as yet no solution is in sight. The plan is to update the version of the installer tool that they’re using for SDA in the hopes that this will solve the problem, but at the present time there’s no known workaround, and no date for a new SDA release.
Indeed, there has been no new release of SDA within the last five days, and I would not expect a new release in the next five days either.
You can download and install all required components for Dorico and our other products from our web site, so for the time being you will need to continue to use that method.