I have finally solved this issue. My plugin checks for the presence of a valid license file on initialization. If a valid license file is not found, it pops up a message box telling the user that it will be runnning in trial mode. The problem seems to be that, because auval is a command line program, it crashes when the plugin creates the message box. It appears that Logic Pro spawns a copy of auval to scan its plugins, which causes validation to fail. This also explains why Reaper doesn't have this problem, as it does not call auval. Ifter removing the popup, validation works perfectly.