To me this sounds like the snapshot is not completing as you said the remove snapshot task is queued, meaning the create snapshot task is still running.
I'd try a few things:
1. Login to vCenter as an Administrator and test can you take and remove a snapshot of the VM manually?
2. Are you selecting the VM Backup Job to capture the memory file as well? Try removing this from the backups
3. Try logging into vCenter as the service account Data Protector uses to connect to vCenter and test creating snapshots manually
4. Do a test (not for long term use) and gave the Data Protector vCenter Account the Administrator Role and test again manually creating snapshots
5. Run the backup job again with the account having Administrator Role in vCenter
6. Import one of your ESXi Hosts directly into the client list using the root password and run a backup direct from the host, rather than via vCenter. Does that work?
That's where I would start.
Chris