The web site is almost back to normal now. WPG2 seems to work fine. I just needed to run the updater for Gallery 2 again with WPG2 and the vSlider3 theme (Gallery 2 version) deactivated. I then reactivated WPG2 and the vSlider3 theme in Gallery 2, and the integration started working again.
For some reason, the light box plug-in no longer works correctly in Gallery 2. Also, the description bubbles on the Gallery 2 version of Google Map (which shows a photo thumbnail when you hover over the map pin) also seem to have some formatting problems. Perhaps I’ll find solutions in the coming days.