Gtk+ Broadway backend application in Windows-7 does not accept Alt+Gr characters

Question

I have an embedded device which is running a Gtkmm application (on an Open-Embedded platform). The Gtk+ Broadway backend of the application runs perfectly fine with all the HTML5 enabled browsers (IE11,Chrome37,Firefox27), when accessed within the network. But the problem is when I try to write an Alt+Gr characters (like the Euro or at"@" symbol) from a German keyboard into any of the Text-fields in the application, I see nothing, it does not accept any character. There is no problem with typing in other characters. Could it be a browser or application specific problem? Actually I have found no information regarding this. Any help would be greatly appreciated.

Thanks !!