This funny thing:

Warning: invalid .mplay_lock file detected
Warning: invalid .mplay_lock file detected
mantra: Could not open imager

still happens in v9.5.169 (Vista x64) after you install Houdini and try to render for a first time.

Go to Render menu in view-camera floating window and set it to “Disconnected”. It will render.

Despite next time you'll start Houdini this option will be set back to “Listen to Renders”, it will render.