I think i had the same problem recently, when trying to open projects done in 5.5 (32bit), then opening them on my 5.5 64bit.
I have a dual boot XP/Win 10 now.
What i done was opened the older project in 32bit/XP, and re saved the full project in a different location via the File/Save Project option… it might be called “back up project”, sorry i can’t remember exactly the terms, but it’s in the first drop down menu top left.
Projects then opened up ok for me.
You could also try opening the actual Cubase file from file explorer and see if it grabs everything in your old project directory… the extension for the project being *.cpr
I think i also tried setting the “set project folder” inside the old/original project folder, and i think it might have worked also, but may have had to then locate all the files etc that were originally used… not 100% sure.