Re: "FAILED: Unable to create a VSS snapshot of the source volume(s). Error code: 2147754758 (0x80042306)." FAILED at 1% with standalone converted 5.5

Your problem is with VSS, probably it is lack of space, check for similar problems and solutions.

The errors you see are expected, we need a full log bundle for investigation, and don't warry about GPT disks, they are supported with Converter 5+