VirtualChannelWrite hang mstsc.exe

I have developed client and server dlls for exchange data with virtual channel.

The dlls use static virtual channel api.

When on server side there are windows server 2003 - 2008 and on client side windows xp, 7, 8 all work fine.

When on server side there is windows server 2012 r2 and on client side windows xp again work fine.

The problem is when on server side there is windows server 2012 r2 and on client side windows  7 or 8.

In this configuration transfer file from server to client work, but when I try to send data from clien to server the client rdp (mstsc.exe) hang, not responding,the onlything that left todo is kill it from task manager.

Debugging client code I saw that the client hang when I write data in the channel (gpEntryPoints->pVirtualChannelWrite).

Any idea?

Thanks, Claudio