VirtualBox Guest Additions: copy-paste works only in one direction

In short:
VirtualBox provides a mechanism to copy/paste clipboard data between host OS and guest OS.

Host: Windows 7, x64, VirtualBox 4.0.4 (should work with any host OS)

Guest: ReactOS 0.3.13 + VBox Guest Additions 4.0.4

I tried Copy-paste (Host<->Guest):

Paste work, copy doesn't. I.e. you can paste data from host to ReactOS guest, but not vice-versa.

If one direction is working, I expect that other should work too...

Needless to say it works on all supported types of real Windows (2000/XP/7/...)

Can someone look into it?

Thanks in advance,

-Technologov, 4.4.2011.