Ola Hallengren DatabaseBackup error on locating Windows directory
Community wiki answer:

Use a domain account with access rights on the share, then this should work. NT_Service will not be able to reach it.

If using a domain account is not possible for some reason, you can try granting the machine account permission to the share. The machine account follows the following format: DOMAIN\MachineName$. The $ is needed to denote it's a computer and not a user.