Error: Instance XML File does not exist!

1. What is the problem? Be very detailed.
Four instances have a "Sending Failed" error in the Send Finalized Form section. On selecting them and clicking "Send", the following shows: "Error: Instance XML File does not exist!".
All the other instances have been sent successfully (about 35 of them).

2. What app or server are you using and on what device and operating system? Include version numbers.
I'm using Kobotoolbox. ODK Collect runs on a Motorola phone running Android version 6.

3. What you have you tried to fix the problem?
The phone is in a remote location and I don't have access to it.

4. What steps can we take to reproduce the problem?
-See above-

5. Anything else we should know or have? If you have a test form or screenshots or logs, attach below.
This exact combination of form, phone, server, user has been running perfectly for close to two weeks, it's just now that this error has shown up.

What version of ODK Collect are you running? Are you also running KoboCollect at the same time? If so, what version?

Is it possible the enumerator accidentally deleted the XML instance (e.g., by using Collect or a file manager)?

Hi amalm!
Did you get the solution?
Facing the same error here...