Update on this issue.
It seems 2.11.7 resolves the original problem when you couldn't delete the cable which terminates a circuit.
But with 2.11.7 you now can't re-add a new cable with exactly same A-end / B-end as deleted one.
(think of scenario when you accidentally deleted the cable and want to add it again)
To reproduce the issue:
1. Add a circuit
2. Add circuit termination Z side ( I didn't add Termination A side)
3. Termination -> Connect -> Interface. Fill in details (lets say Switch1 and Port1)
4. Once cable added, try to "disconnect" it (this should be successful)
5. Try to add the cable again and connect it to exactly same device/port (Switch1 Port1)
6. Netbox throws this error:
Cable matching query does not exist.
Python version: 3.6.8
NetBox version: 2.11.7
When you re-add the cable, specify another port on your device , e.g. Switch1 Port2
Then delete this cable.
Add it again, now with your original port where you want it to be, e.g. Switch1 Port1