I am having this problem too. There are loads of error reports like this one here, and most people have enough space on iCloud and have enabled Mail for iCloud in both places.
What I generally do as a developer when I see a generic user facing error message is to try to find the underlying error, and in this case it may be these:
error 14:59:19.921148+0200 Mail An error occurred uploading Images.zip : CKPrettyError Domain=CKErrorDomain Code=16 UnderlyingError=CKInternalError Domain=CKInternalErrorDomain Code=3002
error 14:59:19.921339+0200 Mail Unable to remove temporary asset at <private> because of error NSError Domain=NSCocoaErrorDomain Code=4 UnderlyingError=NSError Domain=NSPOSIXErrorDomain Code=2
error 14:59:19.922947+0200 Mail An error occurred during upload: CKPrettyError Domain=CKErrorDomain Code=2 UnderlyingError=CKInternalError Domain=CKInternalErrorDomain Code=1011
Can you have someone look into this?
-Axel