NS_ERROR_FAILURE (0x80004005) on Ubuntu 20.04LTS
I was able to solve the problem myself today. The error message was telling me that the version of VirtualBox and the version of the extensions were different.
Go to Preferences > Extensions
Remove the extensions
Download the latest extension pack from https://www.virtualbox.org/wiki/Downloads
Install the up to date extension pack
Now VirtualBox works perfectly again.