Your ac3 file have:
281 frames (8,992 s) of a 2.0 192 Kb/s
+
86843 frames (46 m. 18,976 s.) of a 5.1 384 Kb/s
+
191 frames (6,112 s.) of a 2.0 192 Kb/s
There are many soft than do not accept that mix in one file.
You can use DelayCut to split the file in 3 correct files, or
SplitAc3 to obtain the 5.1 and a 2.0 (first and last part joinned)