VirtualBox in Linux: same virtual machine in a shared folder result in file not accesible every time an user saves the state of the machine

I have created a shared folder in /Users/shared_folder. I have moved the virtual box files to that folder /Users/shared_folder/.VirtualBox VMs/

I have set the whole folder with all the permissions: sudo chmod -R 777 shared_folder. I have switched to the other user, and I have executed the same command. I also have tried with the following command as I read here (without knowing what I was doing by the way): find somedir ( -type d -exec chmod u+rwx,g+rwx,o+rx {} ; -o -type f -exec chmod u+rw,g+rw,o+r {} ; )

Both users can access the same virtual machine... Till some of them save the machine state. Then, the other user cannot access because of not enough reading permissions.

How could I share the same virtual machine with both users?

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