If Claws Mail is running, you can’t use mailto:-links or bring Claws Mail to front using other processes that call Claws Mail via the registry key HKCU\Software\Clients\Mail\Claws Mail\, for Claws mail ignores them, once started.
This commit fixes it: 2012-11-08 [colin] 3.8.1cvs116 * src/common/socket.c * src/gtk/gtkutils.c Fix socket communication