A couple of things I see here, that you may try.
NFS is a bastard, just by the by.....
Firstly, you state that you have other files mounted on this machine. I would unmount them, as it seems though you are mounting / of that box - which presents a sort of weird conundrum - you have / mounted, but then you also have other dirs deeper in the FS mounted elsewhere - it's a lot for RPC to keep tabs on.
Secondly - omit the "-t nfs".
Mount can determine you mean NFS by the presence of the colon.
Lastly - do you have an entry for / in your /etc/exports file on the server? Does exportfs -a exit 0?
I would umount - on the client - all that you have NFS mounted from that server.
On the server, do an exportfs -ua
Then, remove all but / from the /etc/exports file (leave the perms - rw,root=*, etc....)
Rerun exportfs -a
If that doesn't work - exportfs -ua