If your browser shows red warning on start by using HTML5 client

It means that your browser does not support Websockets or your browser does not support Canvas.
Such browsers that do not support these technologies are IE6, IE7, IE8, IE9 and need FLASH to emulate these features.

Browsers supporting Websocket and Canvas

Also see: What’s the difference between websockets and XHR?

If your browser automatically reloads the pages to HTTPS address

It means that Websockets transport is not supported. This is the case on several Android native mobile browsers.
It is due to the fact that the browser automatically switches to XHR transport.

But because this transport layers on long distances with HTTP protocol, each request creates new connection. It is very slow and unstable to create new connections (up to 20 new connections per second), therefore to avoid this instability the program is conceived so that the page reloads automatically to HTTPS address to enforce HTTPS connection.

Physically you also get mostly persistent secured connection and much more stable on long distances. So the logic of browser reuses already established SSL connection instead of creating a new connection like by the use of HTTP protocol.

This behavior can be changed by the following option on the setting.js file located in the RDPlus program folder under this path: Clients\www\software\html5\settings.js:

forcesslforxhr = false;

But it is absolutely not recommended to disable the use of SSL in XHR mode.

*If you do not have a file named “settings.js” in the “C:\Program Files (x86)\RDPlus\Clients\www\software\html5” folder, then your RDPlus version is older and this documentation does not apply. Please update your system first or contact support.