Sometimes I get an exception after recompile when trying to refresh 2 or more pages a time.
I found the issue, it's in mojoSiteMapProvider.cs:

public override SiteMapNode BuildSiteMap()
{
    if (rootNode != null)
    {
        return rootNode;
    }

    lock (objLock)
    {
       [init rootNode]
    }

    [...]
}

As checking for null is not under lock(), 2 threads check simultaneously and then try, one after another, to initialize it 2 times. The second attempt throws an exception.

P.S. And why do you create special object objLock instead of just lock(this)?