Here is what IK Multimedia support told me
First Part: Please follow these steps to correct this error:
1. Click the Cog Wheel (Gear Icon) in the upper right-hand corner and check to make sure your selected download path exists.
1.1 If the path does not exist, please select a new path. The default path is inside Documents folder, inside the IK Multimedia folder, and a sub folder titled IK Product Manager.
1.2 If the download path does exists, ensure it has enough space for the downloaded files.
2. From here, find the Installation path and make sure it has enough space for the installation as well. This is usually the main drive of your system (C:// or Mac HD). The system will need enough space for the download and install files.
3. Lastly, make sure you have permissions to read & write to both the Download and Install paths. On Mac, right-click a folder and click Get Info for this option. Scroll down to view Sharing & Permissions. Adjust Privilege levels to "Read & Write".
Second Part: If this does not help, please send us the following with your reply:
1. Inside IK Product Manager, click the question mark ( ? ) in the top right and send us on what your Server and Client versions are.
2. Send us your System Information. Here is a how to do this on macOS and Windows.
Third Part: To get your downloads outside of IK Product Manager, log into the IK Multimedia website and access your My Products area inside your User Area. The Past Releases section under each product will include the latest download.
My current experience:
1.Part: Everything is as it should be on my side, problem still exists
2.Part: ...ongoing
3.Part: I do that, the downloads work but keep stopping many times. This is annoying and very time consuming. Even though I don't have the fastest internet connection (!), other websites (Steinberg, NI,...) run much more smoothly!
Maybe it will help some of you