Can you open the OSM file with a text editor? The second line should be
<osm version='0.6' generator='xxx'>
If this is not the case, please add it.
Reading this wiki page might also help:
http://wiki.openstreetmap.org/wiki/Osm_formatIf
this didn't help, can you give us an extract of the osm file? some
lines at the beginning and some lines at the end will probably be
enough.