Various troubles uploading models to Mixamo

Hello! 
I am creating a game using Unreal and, as I am not able to model charcaters, I use Make Human which is quite easy to use. 

My problem is strange, as sometimes, importing my character to Mixamo works. 
Some other times, during the uploading, my charcacter appears transparents, and some other times, the rigging process will time out and fail, sending me back to the screen where I have to place the circles on my character. I can try again as many times as I want, after an interminably long loading, I am sent back to this screen... And some other times, it will work without problems! 
The way I work is always the same: I create my character on Make Human and export it as Make Human exchange (Without skeleton) to Blender. From Blender, I export it as a .FBX or .OBJ formats, both of them showing the same results on Mixamo. They can work, they can fail, and I don't understand what parameter will decide for a go or a fail in Mixamo's processes... 

Could someone help me? 

Thanks 🙂