How much free space is left? Backup gets very inefficient once it has to thin all the backups to have room.
Copy a large file to the TC to make sure the network is actually performing as it should.
Decide what is more important.. time or money??
Buy a new 2TB replacement..
Clean up the present TC.. wipe the drive and start over.. you can archive off the backup to usb drive if you want to keep it. Gen1 TC could die 2days after you go to the trouble.. unless you have repaired it.. they are EOL.. power supply is a wonder it stll goes.
In fact, I already had the power supply problem and the model was changed once already by Apple. This the replacement Time Capsule.
There is still 56 GB left, so this should not be the problem.
I went to http://pondini.org/TM/D2.html and erased the spotlight index.
Now it's much faster (4-5 mn).
I still have a strange error:
I'll put it in a separate message, for clarity