The main problem you might run into is opening a YW5 project, then adding a new scene to it. The error indicates it's not saving the scene content file.
So, converting to YW7 is safest.
If it were me, I'd copy just the YW5 project file and RTF5 folder into a new folder called 'project name NEW' and then I'd open this YW5 with yWriter 7.
Next, I'd convert it to YW7 format and save it out. Exit yWriter.
Next, I'd delete the YW5 project and RTF5 folder in this NEW folder (not the original, obviously).
And lastly, I'd move the original project folder (the whole thing) into a zip file and put it somewhere safe. Name it YW5 original + the project name.
Finally, I'd rename the 'project name NEW' folder to just 'project name'
That's what I've done over the years with my approx 30 novels, across multiple versions of yWriter.