i have the same issue here but what i did is to make use of the USB 2.0 or esata connection from the thecus box to the server rather than the network.
for my case i can throw data from the thecus with no problem but from thecus to Tape i got problem. on the backup selection list you can view the box but you cannont select the folder inside unless you install the agent which i think is impossible i also not sure wether there is a symantec agent for thecus you can install. anyway just use the usb or sata and attach to your server.

No comment about the access issue - but to backup any NAS device over share level access - you must make sure that the NAS device itself is not running NDMP. If Backup Exec detects NDMP it assumes that RAWS is installed and then has a failure when it does not get the correct response. Without NDMP on the NAS the media server drops to share level access using the remote agent on the media server to do the work.