Unable to open BCP host data-file error
I had the same - somehow a windows or SQL Server update changed something and a previously successful build started failing.
In my case it was access permissions to the folder containing the dat files that was failing.
I could run the bcp from the command line and it worked but it failed from SQL Server.
The SQL Service needs access to the folder, basically.
To test this, try running

EXEC master..xp_cmdshell 'DIR &lt;your-dat-folder&gt;'


and if that fails check your access permissions (and grant as needed).