Since the VirtualBox 6 update, when downloading guest additions for guest systems this error pops up: The network operation failed with the following error: During network request: Unknown reason.
This is a built in functionality of VirtualBox? Let me explain the scenario more clearly.
You have a guest VM in Solus, you enter the device menu and select "Insert guest additions" and the error occurs. It does not attempt to install guest additions for Solus, it aims to install for the guest system (Windows 7 in my case).