Logging this as a reminder, pending a repeatable test case. Seen before, eg. ALF-20079.
As yet unable to consistently repeat. Approx steps:
1. create sync for single file (file gets pushed to target)
2. edit properties on file in target - eg. author, description
3. at some point the pull apparently fails but may recover ... TBC
Once we have a repeatable test, we should review the code around combining changes and handling null (eg. when there are no combined changes) such that 500 error is not thrown !