it’s not really a howto, but it could help others with the same problem.
I have two Nethservers, one for AD and one for VPN, running as VM on Proxmox. If I restarted Proxmox, my users at the AD were all gone. Restarting the Nethserver with the AD helped to get them back.
The solution of this is very simple, at Proxmox you have to set a start delay for the AD-VM. I use 30 s. Of course the VPN should have a longer delay, if it is part of the domain. I think the problem is a missing network connection after reboot. It takes some time to connect.