I encountered this same issue, where the function name had the leading underscore that caused a blank page when loggin in. However, after fixing that problem and logging in, I am getting an error on my main page that says "You do not have permission to view this page"
I have not been able to find a solution to this. Do you have any idea what is going on here? It appears that the LDAP isn't syncing the groups at all anymore. My guess is that is an issue with the ldap_sync_groups function in userslib.php, but I can't confirm this.
Any help would be great!