Hello Victor,
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.
Thank You
-Team Sellacious