"There was a sync problem." when trying to sync users

We are attempting to sync two sites but are being met with a "There was a sync problem." error after connecting and attempting to sync the first sub-site.

I have cleared options and reset everything, however the problem persists. I have two staging sites where the issue is replicated, I have debug logs enabled however nothing obvious seems to show up in the logs. Can I please have some assistance in troubleshooting? Thanks.