Since I've seen this error for all backups and pointed you to the KB article, I decided to go ahead and follow their process (which I hadn't up until now). The permissions on my registration folder all line up as they say it should. I went ahead to the referred answers.microsoft.com thread and, after making a system restore point, executed from an elevated command prompt:
from szz743's posted step number 4.
sc sdset MSLLDP D:(D;;CCDCLCSWRPWPDTLOCRSDRCWDWO;;;BG)(A;;CCDCLCSWRPWPDTLOCRSDRCWDWO;;;SY)(A;;CCDCLCSWRPDTLOCRSDRCWDWO;;;BA)(A;;CCDCLCSWRPWPDTLOCRSDRCWDWO;;;SO)(A;;LCRPWP;;;S-1-5-80-3141615172-2057878085-1754447212-2405740020-3916490453)(A;;CCLCSWLOCRRC;;;SU)S:(AU;FA;CCDCLCSWRPWPDTLOCRSDRCWDWO;;;WD)
I then ran (right click, Run Now) an incremental backup from my backup schedule definition and it completed fine - log shows it did NOT throw the 513 error this time.