Time Capsule: "There was a problem connecting to the server"
I experience issues like this from time to time. Although running a later version of MacOS (10.13), most of the steps should be similar. 

What I normally do is:


Use AirPort Utility to check if the firmware on the AirPort device is
current, then restart (even if the firmware is up-to-date. 
Run Disk Utility on the Time Capsule backup volume (whether external
or integrated in Time Machine) 
Restart the main computer, holding
down the CMD + R keys to start up in recovery mode, then run Disk
Utility on all storage volumes that are backed up with Time
Machine. 
Disconnect the Time Capsule volume, then reconnect. Hold
down the Option key, then under Time Machine in the menu bar, run
Verify Backups.


Situations like this can be frustrating, and it may take some time to do this, but I find it usually helps. For some other tips, Apple has some suggestions here.