RDP session fails if two(multiple) monitors are used or if the session has very high resolution (e.g. 4K).
SPS usually negotiates session attributes like screen size or color depth with client and adjust the desktop resolution accordingly. In case this is not possible and these limits are exceeded, such connection will automatically fail.
RDP-specific settings on SPS side ("Maximum display width", "Maximum display height") have insufficiently low values to accommodate the requirements of multiple-desktop session.