Synology and Active Directory - "no logon servers available"

I have a Synology Diskstation DS412+ here configured for Active Directory authentication on the shared folders. I have a shared folder that our backup software writes backups to (so, CIFS).

This all works fine on a normal day-to-day basis. The backup software uses a dedicated user account and everything just works.

However, on a monthly basis I patch and restart our domain controllers. Only one at a time is rebooted to maintain Active Directory and not cause interruptions. As a side-effect of all this, when the backup software accesses the folder next, it gets the error There are no logon servers available to service the logon request and fails. When I access the UNC path, I also get the same message. When I reboot the NAS, service returns to normal.

My question is, how do I fix it so this error doesn't occur to begin with? Rebooting is not the best solution as it causes downtime in our cluster services which we're working on moving more on to.

Edit: even stranger, my admin session (which is using my AD user) works fine.

When you receive the 'No logon servers available' it means that your DC controlling Active Directory cannot be found. Make sure that your DC is visible on the Network and that you reboot both the DC and the Synology box. The DC needs to be rebooted before the Synology reboots, otherwise it won't refresh on the network to see the change that appears.

If this fails, check your physical connections and obviously issue a ping and see if it gets to it. If that fails, check the security across the shares and also check the credentials you are using to access the shares.

The problem you do when you restart your DCs is you are not permitting the refresh to occur and thus the backup won't see it until it is last in the restart chain, so to speak. Usually you wouldn't reboot the DC unless absolutely necessary.