As the developer is taking no interest in this unless he's paid for support here is a solution that worked for me.
Log in to your CPanel and go to phpMyAdmin
Locate the appropriate databse
Export the tables that contain your ratings, such as:
Go to the WordPress dashboard and then to GDSR / settings / database maintenance
Delete all database tables
Then to / Remove settings and remove all plugin settings and all the saved widgets.
Then to / Full Uninstall and click Uninstall. Note that the plugin does NOT become inactive after this as stated so then go to
Installed plugins and delete GDSR
Perform a clean install and go back to phpMyAdmin and import the tables exported earlier.
After all of this go to Webmaster Tools and you should immediately see your stars reappear in Rich Snippets Testing Tool along with all of the votes you previously had.
As I posted here and on the GDSR website, this error popped up in some of the websites we run GDSR on after one of the maintenance updates recently.
This is the second time we've had to do do a clean install like this after a maintenance update - the first time we lost hundreds of review ratings.
Give it a day or two and then check your server error logs to see if the reinstall stops GDSR throwing errors.
Hope this helps others.