Error with embedded firefox and proxy

Hi,

if I configure an embedded web browser session, set Firefox as the browser and set a proxy, then no content tab becomes visable. RDM tries to start the session, it shortly changes the button "Open Session" to "Close Session", but nothing more happens.
The configured URL is a http-url (no ssl). The proxy and the URL are working in a normal browser window. It makes no difference if I choose MozNET R19.8 or geckofx. RDM version is 11.1.0.0 on Windows 7 64 Bit. This error happens with the 32- and 64-bit version of RDM. Firefox version is 45.0.1. Antivirus is disabled. Team Viewer is not installed on the system.

Output of the debug log:
Starting process
PID: 9732
MainWindowHandle: 0
Wait 100ms
Wait 100ms
Wait 100ms
Wait 100ms
Wait 100ms
Wait 100ms
Wait 100ms
Wait 100ms
Wait 100ms
Wait 100ms
Wait 100ms
Wait 100ms
Wait 100ms
Silent: System.InvalidOperationException: Process has exited, so the requested information is not available.
at System.Diagnostics.Process.EnsureState(State state)
at System.Diagnostics.Process.get_MainWindowHandle()
at Devolutions.RemoteDesktopManager.Frames.Embedded.FreEmbedded32Cmd.ProcessVisibleChanged()
Process has exited.

Is this a known problem and what can I do to resolve that? Do you need more information?

Best regards,
Andreas