Well i've finally managed to do it!
Here's how to do it, for those who would need it.
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SPSite oSiteCollection = new SPSite("Your site collection");
SPWeb oWebsiteSrc = oSiteCollection.OpenWeb();
SPList oList = oWebsiteSrc.Lists["The name of your list"
];oWebsiteSrc.Site.WebApplication.FormDigestSettings.Enabled = false;SPFolder folder = oList.RootFolder.SubFolders[root.SelectSingleNode("/my:myFields/my:Approvals/my:Departments", NamespaceManager).Value].SubFolders.Add(formName);
SPListItem item = folder.Item;
item.Update();
oWebsiteSrc.Site.WebApplication.FormDigestSettings.Enabled =
true;
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Note that in this line (SPFolder folder = oList.RootFolder.SubFolders[root.SelectSingleNode("/my:myFields/my:Approvals/my:Departments", NamespaceManager).Value].SubFolders.Add(formName);)
if you want to create a folder directly in the list (not in a folder of the list), you can scrap "SubFolders[root.SelectSingleNode("/my:myFields/my:Approvals/my:Departments", NamespaceManager).Value]".
I hope it is clear for everyone!