the issue is most definitely that the SQL service is running as a local account, and not a domain account.
SELECT * FROM sys.[dm_server_services]
when you run any TSQL that accesses a resource outside of the SQL Instnace and it's databases(think \\incpath, drives on the server itself, etc.)
the service account running SQL server must have access to the resource.
it doesn't matter that suser_name() is both sysadmin and domain admin or anything, access to an external resource is via the context of THAT service account.
this is common when someone puts a file on their own desktop on the server, for example, too; the service account doesn't have access to C:\Users\Lowell\Desktop or C:\Users\Lowell\Documents either.
move the file to a location the service account would certainly have access to(SQLDataRoot,backup directory, etc)
or grant the computer resource YourSpecificServerName$ (note the dollar sign! to the\\AppServer\Data\ folder with at least read permissions , do the migration, and then remove the permissions
the screenshot below has a server named Mango as the example:
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