[The Dougfather]

Yeah, i m a night owl.

The whole Pro Tools session should be encapsulated inside one folder so you'll want everything in that folder if the session is to remain completely intact.

The save copy feature will allow you to ensure you have everything and not miss something that may not necessarily be in the session folder.

Check out the general troubleshooting sticky at the top of the forum with regards to the error it could be dozens of things.

Off the cuff if you have more than one processor in the playback engine reduce it by one so if you have two drop to one, four drop to three. Also check all your third party plugins are compatible with your setup irrespective if they're being used in the session or not, they can still cause these types of issues.