While upgrading vRops 6.0.1 to 6.1, the OS upgrade went smoothly, but the application upgrade kept failing at step 4 of 9. Rolling back to the pre-upgrade snapshot and repeating the process replicated the same failure multiple times. VMware has released a KB article to resolve this issue.
To resolve this issue on my cluster, i had to login to the master node as root, and perform the following commands:
cd /storage/log/var/log/vmware/vcops mv ./vcops_upgrade_state.json ./vcops_upgrade_state.json.old
Once this file was moved, the upgrade than proceeded without issue and vRops 6.1 was installed successfully.
For official steps documented by VMware, see KB212735