My functioning export from my testing root mount used async rather than sync. There was also an issue at one point that caused problems when Gb network was set for high mtu/jumbo frames. Is that your exact export line? Because yours also doesn’t include the netmask designation (/24).
/media/raspbmc 192.168.0.0/24(rw,async,no_root_squash,no_subtree_check,insecure)