LHC@home

I'm a Ubuntu 18 user and a few weeks ago, i've upgraded Boinc to the latest version (from 7.16 to 7.18.1). And since then all wu using virtualbox are crashing.
Here is an example : https://lhcathome.cern.ch/lhcathome/result.php?resultid=342677590

Running this command

sudo -u boinc VBoxManage -q list systemproperties

gives me the following result :

VBoxManage: error: Failed to initialize COM because the global settings directory '/home/bigcheaf/.config/VirtualBox' is not accessible!

So it seems to be a permissions issue.
I've checked and here is the summary of the situation :
VirtualBox folder : owner = bigcheaf, group = bigcheaf, permisions = drwxrwx---
"boinc" user belongs to the "bigcheaf" and "vboxusers" groups

Finally, this command :

sudo -u boinc namei -l /home/bigcheaf/.config/VirtualBox

results in :

f: /home/bigcheaf/.config/VirtualBox
drwxr-xr-x root     root     /
drwxr-xr-x root     root     home
drwxrwxr-x bigcheaf bigcheaf bigcheaf
drwxrwx--- bigcheaf bigcheaf .config
                             VirtualBox — fichier ou répertoire inexistant

I can't figure out where the problem is and how to fix it.
If anyone would like to help me...

VirtualBox — fichier ou répertoire inexistant

There is no virtualbox directory in there, create one.

There is a Virtualbox directory :

sudo -u bigcheaf namei -l /home/bigcheaf/.config/VirtualBox

f: /home/bigcheaf/.config/VirtualBox
drwxr-xr-x root     root     /
drwxr-xr-x root     root     home
drwxrwxr-x bigcheaf bigcheaf bigcheaf
drwxrwx--- bigcheaf bigcheaf .config
drwxrwx--- bigcheaf bigcheaf VirtualBox

But the "boinc" user have no permissions on it.