I have found a message from flop25 [19-Jan-2014] which says ....
If you have an error after the update:
- reupload all files of that new version, by ftp and by overwriting existing files

I would like to give this a try, BUT -
1, where do I find the files of that new version? - at Piwigo?
2. what is the FTP hostname? - the full domain name?
3. where do I find the existing files that I need to overwrite on my site? - what folder/directory would they have been saved to on my site's public_html directory?

Comment: if this (system reload) is a usful process to sort errors, please could we have such a process in the Maintenance Tools?

I have run the following from the Maintenance Tools -
1. repair and optimise database
2. reinitialise integrity check
and I have done a synchronization firstly as a simulation and then for real.
These actions have made no difference - the problem still exists.

I have been doing an analysis on those albums that error on being selected for display.
As there are 582 albums on the website I have taken a sample: those with identification numbers between 1 and 50.  The results are that 17 albums open satisactorily; seven are 'not found' - being deleted sometime in the past; and 26 albums show the error.
I wanted to check this high proportion of failing albums and so set about with the smallest section of the website, under the name of 'Roads'. Out of the total of 60 albums in this section, only 15 failed to display.  However I could not find a reason why some ablums in this sample correctly opened when selected and the others did not.
Albums can be selected for display in a variety of ways/from different points in the Piwigo design.  In all cases the album was consistent in opening or failing to open, whichever way was chosen to open the album.