Well, just a guess. A2DSource tries first to use AVISource to open the file, if it fails it shifts to DirectShowSource and finally to FFMpegSource. Probably the first AVI (at least the beginning of it) goes thru AVISource but after 50 seconds it quits. Maybe a not properly encoded XviD.
You can confirm by selecting AVISource as default and then try to convert again the "problematic" XviD. Just to know, where did you get it?
Bye