I managed to diagnose the cause of the error and fix it. I had created two new text fields in the items table:
- g2_summary to display what was description text on the album thumbnails, as in Gallery2, and to split it off from the Gallery2 description text at the request of the client. (The g2_summary field was added to the search index also).
- tag_album was set to album when adding albums, and allowed filtering of custom tag albums (the root tag album, and a custom recent albums tag album) from the albumtree by making changes to the albumtree code, again at the request of the client
The cause of the error of no photos being added (on this new server) was diagnosed by reviewing the log files in the var folder - errors were reported in log files when adding photos as the g2_summary and tag_album fields did not have default values defined in the items table. Setting the default values of both of these fields to NULL removed the error and the customised Gallery3 installation on the new server now seems to be working fine.
My remaining issue is in understanding why this error did not occur on the original server, when these two new fields also did not have default values in the SQL table.