Damn you know I never thought of that.
I converted a couple of files yesterday on my GF's computer, they showed up perfect on the PC and when I sent them to the Sansa, they played fine too.
So I queued up a whole bunch of avi's on my fast new PC this afternoon, batch converted them all and when I could not play any of them on my PC. figured something was bad about the conversion. Lo and behold, after reading your message I copied a couple of them over to the Sansa and they are fine.
It must be that there is a codec on her machine which I don't have. Doesn't seem possible as we both use KMPlayer with (I thought up till now) its default codecs, but that has to be the problem.
Thanks!