After installing the web client try and login.
If this fails then install the PureCM client and check you can connect to the server with these details using the GUI.
If you can connect using the GUI but the web client is still failing then you want to verify that you can connect to your server using the .NET PureCM API.
Download TestPureCMSDK.exe. Copy it into the bin folder of the website. Run it. Check that you are able to connect to the server. If you are not able to connect to the server then this should give a more descriptive error message.
If there is an error about being unable to load a PureCM dll then download Dependency Walker and check you have all the dependencies.