Error: Address Span 24MB combined with 8MB

That 24M address span is really a 32M space. So that 8M location can be placed at 0x42000000 to avoid this.

Like HJS said, spans are usually powers of 2. In the case of that 24M slave port they probably added a property that tells the tools that only the first 24M of the 32M span is valid. So if you shoved the 8M slave directly after the 24M slave then the decoder will not work properly which is what the tools are trying to tell you.