Thank You for reporting this issue. Yes, this issue was because you're using helix ultimate and advanced SEF. However, we have fixed this.
To fix this issue go to components/com_sellacious/controllers/order.php
and go to line no. 318, 325 and 333 or search for . '&layout=completed'
and remove it.
Same with . '&layout=failed'
and . '&layout=cancelled'
in line no. 338 and 343.
I hope this helps.