You are encountering this problem with web credentials: 

You enter no web password and the "allow empty password" option is enabled, but it is impossible to connect from the web portal.

No error message is generated, it is as if the "logon" button does not work.

Check the source code of your Web Portal page, maybe you are using either Load Balancing or Assigned server.

"page_configuration["is_standard"] = false;"

When a request is done to your TSplus server, maybe both are deactivated.

From your AdminTool\WEB\Web Portal\Web Portal Preferences, please uncheck the "Generate a Gateway Portal enabled Web Page" :

Note that this solution may solve the problem of "when you try to connect through the web portal, but you have an" invalid credentials " error message.