Hey there,
When creating a task using the Mirror template, the UI lets you pick a folder from the Explorer with the ... button.
As a result, it will append that absolute path to the target parameter of the robocopy command, resulting in something like robocopy D:\source \\123.456.789.101D:\target, where you actually want robocopy D:\source \\123.456.789.101\target.
I'd say the UI should not let you pick a folder from the explorer, but rather only allow to type a path.