Same here with a QNAP NAS. Same config as bubbadss. Checked on QNAP forum, no success.
Here's what I get: The "***.sparsebundle" file is created in the share directory, which probably means Time Machine can access the smb shared directory.
Some files are loaded in the image file, with a fresh time stamp corresponding to the time of the backup.
After a few minutes, I get the dreaded "the backup image could not be accessed..."
...I wonder if it could be a timeout issue?