Upgraded to 2018.2.3 to see if it would resolve the problem. The backup still fails in the same step, but it does run much longer before failing (maybe that's a clue). Anyway. I opened a case with support. I will report back when the problem is resolved.
I'm having the same issue at the same step " Assembling the tsbak archive failed". Were you able to resolve this? If yes, could you please share.