Hopefully, there is no problem per se upgrading from a v1.5.2 image to v1.5.3, but it is possible some config file got e.g. overwritten along the way (via dispatch-conf) leading to the issue you experienced.
Please make sure that you have "Manage layout: globally" set for the xfce4-xkb-plugin (the flag-icon panel plugin), and then yes, you can try using the screen lock (I assume this is the X screensaver lock?) to see if the password works.
In the meantime, if you used PINN I believe that yes, you can just re-install one OS on your card, leaving the others untouched, but please double check this with procount first - you can post a query on the main PINN thread
(he usually responds quickly).