Revisiting - System.IO.IOException: The specified server cannot perform the requested operation went through a rough phase where I cited the link I gave to you, plus lots more on the history of this challenge. Finally the tradeoffs were analyzed, and the following was chosen. Maybe the same plan works for you?
I’m not sure what other steps are needed. It might need some help, but at least it could access the share.
EDIT:
Service install - Backup to a network share (ie. FreeNAS) may also be of interest, and explains how using ordinary user account for the service can get into a password change issue for the service if the user ever changes passwords. If this is a problem for you, information is available on the Internet on how to handle it.