Import-PWDocuments usage. "Valid default storage area OR ProjectWise folder path must be provided"

I am trying to use the Import-PWDocuments cmdlet to create/update documents (pwps_dab 1.25.1.0). 

I simply want to import all the content of a folder on my local drive to an existing folder in ProjectWise.

The first time, if the document does not exist, it should be created in PW. If it exists, if should be updated. No version is needed here, a simple overwrite will do.

Take this example:

FROM: C:\Temp\Samples\ImportFrom\

TO:      pw:\\<server:datasource>\Documents\Test-Project\Samples\

After connecting to the datasource, the script looks like:

$inputFolder = "C:\Temp\Samples\ImportFrom"
$pwFolder = "Test-Project\Samples"
Import-PWDocuments -InputFolder $inputFolder -ProjectWiseFolder $pwFolder -ExcludeSourceDirectoryFromTargetPath
However it fails with the error ("warning"):
WARNING: Valid default storage area OR ProjectWise folder path must be provided
I have no idea what I am doing wrong and find this message rather confusing.
Hopefully someone here can help.