I'm pretty shure i have play'd mp4-files in realone, but now i had an upgrade, and i wont play them... do i remember wrong or what? If i install an older version of realone, will it then be able to play mp4-files? or is there an other app, that will play mp4, from MMC?

yes it should play mp4
I sent one mp4 from my brothers P800 on to my 3650 and it played it
but i sent another mp4 it dident play
i think its because of the encoding trying encoding the mp4 again like you did before when it woked, it should work

It doesn't play all mp4s, i encoded one and it worked, another one was simply refused. :-?

My real1 cant even see the file

ok... now i know why... My real1 couldn't detect the file, because my MMC wasn't formatted on the phone.

It doesn't play all mp4s, i encoded one and it worked, another one was simply refused. :-?
yea exactly same thing happened to me

But it really pisses you off if it's a huge file, and you've been waiting for a long time to put in via IR, then poof! It just doesn't work! :P