you have the error in the logs you posted.
default: The guest additions on this VM do not match the installed version of
default: VirtualBox! In most cases this is fine, but in rare cases it can
default: prevent things such as shared folders from working properly. If you see
default: shared folder errors, please make sure the guest additions within the
default: virtual machine match the version of VirtualBox you have installed on
default: your host and reload your VM.
Then it tries to to update the VM but, fails because its already getting updated on the inside. I go inside the VM and do apt-get upgrade then do a restart. The shared files never mount.