Hi, Sondos-
I think that isn't an error or anything, but a sign that there was no direct connection found between ROI1 and ROI2; there were apparently connections found between pretty much all of the other possible pairs in that case (a large amount of interconnectedness which I haven't really seen before, but can be OK in principle).
If you look at the *.grid files, my guess is then that in the "NT" (="number of tracts") matrix will have a 0 in the ROI1 column and ROI2 row (and equivalently in the ROI1 row and ROI2 column).
--pt