Hi all, I'm encountering the same issue on an Acquia hosted site, only on my production environment. I'm not totally sure whether my problem is the same/similar as what's mentioned above about Pantheon sites, but did find this snippet on the following page:
https://docs.acquia.com/acquia-cloud-platform/manage-apps/files/temporary
"Drupal modules that write to temporary directories may require additional configuration to function properly in environments with multiple infrastructures, such as production environments for Cloud Platform Enterprise and Cloud Platform Site Factory."
It then goes on to list modules that are incompatible with Cloud Platform, but Single Content Sync is not mentioned. I'm thinking the patch mentioned above would help me as well.
Thank you so much for this wonderful module -- my team has been wanting this exact functionality for a while and it's been quite useful already on our non-production environments!