Hey medlo i read your post in maniaplanet forum but respond here.
I think the problem is that the game saves the titlepack in which the replay is driven and when its different from the titlepack in which the map is build then MX does not accept the replay. Btw the error message is misleading its says something like "replay does not use caynon car" which is not right instead it should probably be something like "the titlepack is different". With the program GbxDump you can view this information.
However i found a workaround for this problem. I have done it with my replay on your map "
Tamacun by medlo.
Its originally driven in the tm2 titlepack and i could not upload it. Then i opened the normal canyon titlepack and did "Export to validate" with replay editor. Now i could upload the replay. However such replays are not viewable the only thing you can do with this is validate. IMO this is not good. I always thought you can only upload ghost replays to MX.
Edit: when you just open the replay and save it is also not possible to upload.