The error you're describing normally only occurs when something else (such as a network content firewall you might see in an education or work environment) is blocking a file from being downloaded. The solution is to finish the launcher download outside that controlled environment (so use another Internet source, or a VPN).
You can enable logging in the launcher settings if you want to figure out precisely what's being blocked, though the solutions above should workaround whatever is being blocked. Logs are saved inside GameData\MBII_Launcher (you may want to check/sanitize private info first if you plan to post it here, since it can contain part of your local computer's username depending on the install location).