If your issue is anything like mine then this is how you kill the connection and fix the problem:
1) From another computer connected to your LAN, log into your NAS from a web browser.
This might be different depending on how you setup your NAS. You probably should have connected to the NAS in order to initially set it up so you know how to do this?
BTW, if after entering your login details it tells you 'The operation failed' and it hasn't logged you in, just put the curser in the web browser address field and hit enter - some crazy Synology bug that doesn't happen when I connect from Windoze.
2) Once in the NAS you need to kill the 'afp' connection of the Time Machine login.
If your 'widgets' aren't showing click the widget icon near the top right corner. Under 'connected users' you should see your web-based connection with your username and of the type 'http' - DON'T KILL THIS ONE 🙂 Hopefully you'll see an 'afp' connection that is the Time Machine connection with that users account name with it. Click on the red negative symbol and then agree to kill the connection.
3) Tell Time machine to do a backup now and hopefully it all works.
Occasionally the Synology will not let go of the backup and you will have to repeat this. As long as that 'afp' connection remains the backup shows as in use and Time Machine cannot access it.
Good luck.
S.