i have perfectly same problem,
It solved(mine).... by...
obviously get rid of the custom.... hitsound
which one should be get rid of?
i dont know ... but im certain ONE of the custom hitsound is rejected and prevent the creating preview to be completed
i have around 18 hitsound, i reduce it to 12
uploaded it, failed and stucked
i reduced it again and again till its accepted and the submission complete.
and i think...(IMO)
ur hitsound should be name correctly and doesnt have space in their name
like soft normal2, which should be soft-normal2
and the name must be format of custom sound,
soft-hitnormal
normal-hitwhistle2
normal-hitclap
none any other than available format from osu,
then it works just fine, but its still kind'a frustated, getting rid of something which we dont know which custom hitsound to be getting rid of,
or perhaps u want to try the possibilities ( possible thou ) , try by narrowing the list.... while uploading
and again.... ( might be time consuming )