Q: Time Machine on my new MacBook can't connect to OS X server on another machine.

I have a new MacBook running El Capitan 10.11.6 and up-to-date on all updates. I am using my old MacBook Pro (2009) as a backup server. The MacBook Pro is also running El Capitan 10.11.6. I'm using OS X Server 5.1.7 (which I believe is the latest release). My local network is running on Airport Express, ~2 years old, but has never had any issues and is used by me and my wife daily. We work from home so it gets lots of use.

I have a Time Capsule set up under OS X, and up until about 2 weeks ago it worked without a problem for roughly 3 months.

Then there was a local power failure for a few seconds which shut down the network but not the computers. Now I can connect to the server for file sharing and screen sharing, but Time Machine has no backup disk selected and when I try to select the disk on the server, it tries to connect and then says,

There Was a Problem Connecting to the Server "Mad Dogs Laptop".

Check the server name or IP address. If you continue to have problems contact your system administrator.

Since it doesn't tell me what the problem was, and I am the system administrator, I am at a loss.

The Time Capsule storage is on an external drive on the server, but that drive is accessible both locally and remotely via Finder. Also the server runs its own backup on that same external drive every hour just as it should.

There must be a user name/password somewhere that Time Machine is using, but I don't know why it would have changed and I don't know where to go to check it or reenter it. The same user name and password continues to work for connecting through Finder and Screen Sharing.