Finally I thought it was time to move my Gallery2 configuration + albums to the production site. This time making sure that I took the necessary precautions.
Yes, it was uploaded without errors and yes, I could switch off maintenance mode and yes, I could see my pictures and enter the site admin mode etc.
However, there was one album that didn't show its thumbnail. Rebuilding the thumbnails produced a hang half way through. Editing the album wasn't possible, it produced just a blank page. Editing the other albums worked fine. I could even see the contents of the faulty album if I just selected it from the list of albums shown in the sidebar when viewing another album.
After several hours I found the culprit: the original image I used for the custom thumbnail was 1Mb, 2000+ x 1500+ pixels that had to be resized to 150x150. I didn't notice because it was done smoothly on my laptop. After resizing the 'original' thumbnail to 200x150 and uploading it to the corresponding directory (g2data/plugins_data/modules/thumbnail/<nr>/ the thumbnail rebuild continued until the end. The thumbnail was restored and the album was totally functional again.