Configuring a Dell DR6300 with Veeam 9UP1 on WinSvr 2012 I had this happing. Was resolved by editing the registry and changing the credential format. I used hostIP\sharename for mine. See the instructions below:
These registry values must be created on the the SMB client, which is specified in the repository settings as “Gateway server”. If the gateway is set to “automatic”, add these registry values to all Windows servers managed by Veeam Backup & Replication.
For Veeam B&R 22.214.171.1247 or later, the following registry value allows Data Movers to open backup files by a resilient method similar to the Windows net use command.
Key: HKLM\SOFTWARE\Veeam\Veeam Backup and Replication\
Note: If credentials are required, specify Domain\User or Host\User in the repository settings. .\User credentials are not supported with NetUseShareAccess. For example, specify “NAS-01\Admin” instead of “.\Admin”.