Hi
I am just new to this forum and I found it because I was having the same problems that you describe. My Mp4 player was supplied with ver 3.5.? of AMV convertor and any video I converted would play the first frame then display the message "format error". I downloaded Ver 4.0 from this site which gave me the same results. Then I noticed Ver 3.68, also available for download and it works perfectly. I have been using another bit of Freeware called DVDx which is available at
http://www.labdv.com/dvdx/
withthis peice of freeware you can rip straight from a DVD then use AMV 3.68 , drag the amv file on to your Mp4 player and it should working fine.
It seems to me that the only version of AMV that works is Ver3.68,
Good luck
Magicdj