I'm pretty sure minehut is still using file.io for uploading files, which means that there is a 100MB limit on your world file size. If it is larger than 100MB then it just says 'failed to upload file'. Which, has been a problem for about 3 months. But, what do you expect? This is free .-. (That said, please fix this. I need to download my world too)
If you don't think it would be really big, you could try to download each world individually using /dl world (worlds), and you can see what worlds you have using /worlds