Ah, yes... but that's for a different reason.
That will happen when navigating to nodes like "This PC". In this case, the script will be called with an empty or uninitialized tab or tab.path.
You can test this with IsEmpty() and IsNull(). One test might be sufficient, but I don't know VBscript well enough to be sure.
Alternatively, you could use PathType():
Returns a string indicating the underlying "namespace" type of the specified file path.
The EverythingFolderSize add-in contains many tests to cover all situations. It's in JScript, but you'll get the idea.
Actually, there was nothing wrong with
If Left(scriptColData.tab.path, 6) = "ftp://"
Left() handles the path object with no problem, no CStr() is needed. Sorry!