Best guess it that this was a faulty upgrade. Perhaps a file permissions problem or maybe some files from an older install did not get replaced.
Check all your logs to see what else is shown. Specially the server access log, the CGI error log and the Zenphoto debug log. The only way that you can get this error is for the functions-controller.php script to fail to load. That load would have been the action done immediately before the execution of the statement showing the error.
It works!
After several re-uploads, I managed to have the new version working. It seems that index.php did not overwrite properly. (I know you recommend not too overwrite this file, but to erase it and then upload the new file!!!).
Thank you very much for your help. I always learn a lot from your replies.
I think you misunderstood him. I actually would always recommend to delete all files first and then upload the new ones.
It somehow happens that via FTP frequently that some are not correctly overwritten if you for example drop the entire zp-core on zp-core. I often had problems that way somehow but not the other way. Maybe it is the the number of files, no idea.