On some macOS systems, it is not possible to open the Steinberg Download Assistant at all, or a spinning wheel is shown with no option to access the application.
This error can be caused by a font with an invalid name that is installed on the system.
First, please back up your macOS fonts collection to be able to reinstall the non-standard fonts later, in case this approach could not resolve the issue.
To do so, go to the folder /System/Library/, right-click the contained folder "Fonts" and select "Compress 'Fonts'" from the context menu. This creates a .zip file containing the complete "Fonts" folder as a backup.
Now, restore the fonts that were originally included in macOS using the "Font Book". A description on how this is done can be found on Apple's support pages. Make sure to select your OS version, as the steps to achieve this may differ.
If the Steinberg Download Assistant starts with the standard macOS font set, there was at least one font installed that does not work correctly. If you need to narrow this further down, you can reinstall the additional fonts from the compressed backup file one by one until the error reappears.