Ok, it may not be lost ^^
if by any chance you can open it (without any changes) with Trackmania ;
save it on an other name (without special charaters) . then re-open it in Unlimiter.
If with a Big luck and you can open it; go in your mt work and get rid of all ghosts that use a custom skin.
unlimiter don't like skins and mod sometime.
then save it

We hade this kind of problem before but with CE and TS, so have a look in
those threads, it may help.
But NOM may be right ^^ its maybe easier to try to modify in unlimiter your blocs before this