A conversion task is running normal when selecting a different source VM. Must be specific to that VM.
Details of source VM:
Power state: offline
Compatibility: ESXi 5.5 and later (VM version 10)
Guest OS: Windows NT 4.0 SP6
VMware Tools: Not running, version:7392 (Current)
CPU: 1
Memory: 1024 MB
Hard Disk 1: 8.00 GB
Nothing uncommon except the very old OS. Maybe its that issue?