Hi, sorry for posting here.
I had to use SendMessage winapi call because that's what putty uses internally . There is no official api for pageant. About you problem I can't help you because I do not have windows server 2008 environment to test it. It might be the problem with stricter uac in server edition. Are both you processes running with same elevation level? Here is the link with some fixes that uses new (at leat was new when I wrote it )AuthenticationMethod api. link
I had to use SendMessage winapi call because that's what putty uses internally . There is no official api for pageant. About you problem I can't help you because I do not have windows server 2008 environment to test it. It might be the problem with stricter uac in server edition. Are both you processes running with same elevation level? Here is the link with some fixes that uses new (at leat was new when I wrote it )AuthenticationMethod api. link
Best regards.