Tried to upload a world twice, first time it went to 200% then after a minute it responded with 'upload failed (-1)'. Tried to upload again this time to 100% then the same issue appeared. Every attempt since then this error appears 'Upload failed! (Realms ( Errorcode: 6003 ) ): Upload Limit reached)'.
The world is 26mb and was created in 1.7.10
This is a problem that happened to me too. I looked it up on the web and found that you can only upload 3 worlds every 3 days. The same is for downloading.