If you have a “mobi” format eBook file and want to convert it to “epub” format like me, you can use the software “Calibre” to do it, which is available on Linux/Mac/Windows.
For Ubuntu, run the following commands to install Calibre and convert the format.
There are also online services which can do the same thing like here. You can use such services if you don’t want to install a software to do it.
Yesterday I got the “NS_ERROR_FAILURE (0x80004005)” error when starting a VM instance in VirtualBox. Reinstalling didn’t work and the error was still there. The real problem is the security permission. Go to “System Preferences -> Security & Privacy”, confirm the permission needed by VirtualBox, then the problem is solved.