HELP 5.5.5 Runtime Error R6106

First of all I’ve never seen the error you are getting.
But a few things you could try…

Try to open the project without any plugins loading.
You’ll need to rename your vst plugins folder or move it.
If the project opens without plugins loading,
you’ll need to go through your plugins one at a time until you find the culprit.

Also, trashing the preferences never seems to hurt.

Sorry not more help.