S3-S3 Datasync Fails with error: The endpoint has already opened a volume for this session

I'm trying to clone a number of buckets between two AWS accounts. Of the 6 buckets that are being copied, 2 consistently fail to sync with the error:

The endpoint has already opened a volume for this session

The sync task starts and goes into a "Preparing" state, it then stops here for up to a few hours, then just errors.

The two buckets that fail are the largest buckets from this batch (one is 10 TB the other is 1 TB), but object count looks like it shouldn't be exceeding the quota for a task (12.3 million and 7.6 million respectively).

I've tried various combinations of only synchronising some directories and turning off only synchronising changes, neither has helped. I've also tried completely recreating the synchronisation locations and tasks and this also didn't make any impact.

Looking at the logs, I see a normal startup sequence:

[INFO] Request to start task-xxx.
[INFO] Execution exec-xxx started.
[INFO] Started logging in destination hostId: host-xxx for Execution exec-xxx
[INFO] Started logging in destination hostId: host-xxx for Execution exec-xxx

Then no further log messages are recorded.

Has anyone had any similar experiences with s3-s3 sync? or any ideas what could be going wrong?