UNC paths and windows service accounts are notorious for not working nicely together.
from what i remember the “workaround” was to;
- manually login with the service account on the machine youre running sonarr (you may need to enable interactive login)
- use windows file explorer to access the UNC path
- enter the appropriate credentials and tick the “remember my credentials” option - that will then save those credentials into the account profile.
- then reboot.
theoretically it should now be able to use those stored credentials to access the UNC path.
dont forget to disable interactive login
or you can do that 