Web Client requires the user account to be entered in a domainName\UserID format, otherwise it treats it like a local user account and will state the account is invalid, as it won't find it in the domain security group and/or users specified to have access to Web Client when the installation is done.
For the issue of Web Client hanging on the 'Microsoft Dynamics GP' screen, we see that when Web Client cannot find the Runtime Service, which is used to load the GP application in the browser window.
In most cases, this is because the SSL certificate being used with Web Client and required to be added to the Runtime Service during the installation, is not the FQDN of the server that Web Client is installed onto (or the session host server if you have a multi-server configuration for Web Client).
You can verify the Runtime Service URL being used by browsing into the C:\Program Files\Microsoft Dynamics\GP Web Components\SessionService\ folder where Web Client is installed and look at the SessionService.config file. The Runtime Service URL will look something like MyWebClient.Contoso.local:443
Once you get the Runtime Service URL, you should be able to ping the host name from the certificate, i.e. MyWebClient.Contoso.local, using my example here, from any machine where you're attempting to access Web Client, and have it return the IP address of the server where the Runtime Service exists.
We see this especially with third-party certificates and/or using a wildcard certificate, which usually do not have the FQDN of the server as the host name/configuration for the certificate, thus we'd need something setup in DNS or some type of forwarding from the certificate SANs to the IP address of the server.
Let me know if you have any questions.
Thanks
