I posted earlier about this issue. (Error When Converting Airport) I re-installed the Content Converter and the AC3D plug-in. With only the rwy.tgi being converted, and the .tsc correct this error still occurs. This is the .tm log.
converting folder = './'
0.12: converting geometry file './lind_rwy.tgi'
0.12: (type = 'ground')
0.13: ERROR: (loading geometry './.tgb' failed)
0.13: ERROR: (loading './.tgb' failed)
0.13: ERROR: (unable to load geometry './.tgb' that is given in tsc file './/lind.tsc')
0.13: ERROR: (cannot convert geometry file './lind_rwy.tgi')