Are both panel families the same family? If so you might look at how they are built, or other family elements within that system.
I ran into this issue with walls a while back with some the walls category. While I am not sure if this is the same issue as you are running into, but in my case the issue was actually a nested window family having multiple opening cuts. Once I fixed the nested family it worked perfectly.
There was an opening cut modeled in “Window B” & “Window A” separately, which were used to make nested family “Window AB”. This meant there was not a singular opening cut within “Window AB” and instead separate openings in the original window families. This was causing RiR to throw the error of a one and only one closed loop. Once I remade the family to have a single opening cut in nested family “Window AB” it fixed the issue and worked perfectly.
Anyone else have any other experiences with error and found a different solution?