I can't imagine ext4 can't support volumes of 'only' 8TB...
The ext4 filesystem can support volumes with sizes up to 1 exbibyte (EiB) and files with sizes up to 16 tebibytes (TiB).
But I'm sure you already knew that.
I suspect it has something to do with the size and/or location of your boot partition.
Why not follow the official installation manual? http://doc.zentyal.org/en/installation.html
Or do you have to use custom partitions?
btw.. what kind of raid5 do you use? is it a hardware controller or do you use software raid?