I've had the same problem today where the conversion fails somewhere in stage 1. Granted, this is my first time attempting a conversion from GeoPak to Civil 3D.
I also have had trouble converting .tin surfaces to civil 3D. The translation completes in this case, but does not convert any of the triangles. When I download the translated files, they are all empty, besides the excel sheet showing that 0 of 12633 triangles are converted.
I have seen another post about problems with timing out during translation with too many triangles, but the conversion only seems to take about 3 minutes.