Hi Staffan,
Is your installer an MSI?
If so then parent process (your installer) / msiexec is running as SYSTEM / Administrator. That means that any sub-process it starts will be unable to connect via COM to Plantronics Spokes process that is parented by a process owned by the current interactive user (logged on user).
The mismatch between Admin/SYSTEM and current interactive user prevents COM to work.
Use Process Explorer (running as Administrator, i.e. hold Ctrl-Shift as you run "procexp.exe) to check the PlantronicsURE.exe process properties, the Image and Security Tab (and Permissions button in Security Tab). Compare the fields with the process properties of your application.
See if there are any differences with your application when it is started by your installer versus when it is started manually by current interactive user.
Consider using InstallShield which wraps your installer in an EXE process and is able to start your process at end of install running with correct parent process (that of current interactive user).
Hope it helps, let us know how you get on! 
Thanks,
Lewis.