I have this problem also .. and it's H in box 7 .. direct Roth401k-to-RothIRA. Why anyone would care about cost basis in this transaction is beyond me. Unfortunately, TT had taken this box 5 amount and added to the IRA worksheet as taxable at conversion. My and other Roth401k distribution (1099 H-type) did not put anything in box 5 .. very difficult to calculate what I contributed (to Roth401k) over the years but who needs that anyway. Can I simply choose to not enter box 5 so TT won't get confused ?
