First I hope this question finds all doing well and thanks for any responses.
I have a Windows 2011 SBS and I just migrated my AD from it to a new Windows 2019 server. Promoted it to a DC Transferred FSMO roles everything went well and the AD is replicating between the 2 servers all seemed fine. When I had to reboot the 2011 SBS I noticed the when I try to log a workstaion in when the 2011 SBS is ofline (rebooting) the workstation couldn't login I got this message 
once the 2011SBS is back up the workstations login fine again. Anyone have any ideas why this is occurring,?
Thank you for any ideas.

