Many of the output formats PP has available aren't suitable for reediting, so this may not be all that surprising. The questions is, what exactly did you output?
I don't think I ever output anything- I just did the autosaves and did regular 'save'. I never actually exported it. Does that help at all? This is all new to me!
Maybe. You said you made videos, that means an export. Until it's exported, it's still just a project file, not yet a 'video'.
So...you're saying your old projects give you an error? Try this:
And give this one a read, too. You'll get better answers if you know what things are called.