unable to map network location of sharepoint web application

I have 6 web applications on a sharepoint server.
What happened is, I am able to map a network location with the URL (eg: http://www.exmple.com@portNumber) on most of the web applications except 2. one with port 97 and the other 99.
the error message prompted was "The folder you entered does not appear to be valid".

anyone able to shed some light on this?