My gallery suddenly ceases to work, and automatically went to the setup page I don't know why and refused to connect to the DB with several error messages.
I tried then to upgrade to 1.5.4.2 version but the problem remains: at the beginning of the setup page, I have this message:

Parse error: syntax error, unexpected '' (T_ENCAPSED_AND_WHITESPACE), expecting identifier (T_STRING) or variable (T_VARIABLE) or number (T_NUM_STRING) in /homez.110/marienoe/www/photos/zp-core/setup/index.php(229) : eval()'d code on line 1

Further down on the page, the DB engine is set to MySQLi though it has always been MySQL and I never changed it. And the admin, password and other fields are filled with HTML messages such as
Notice: Undefined index: mysql_user in /homez.110/marienoe/www/photos/zp-core/setup/setup-sqlform.php on line 74

Please advise on how to solve this issue. I didn't change anything, and the gallery was working perfectly last time I displayed it yesterday.
Thanks.

The first error indicates a problem with your configuration file. Probably such a problem was the initial issue with you site as well.

MySQLi will be the default database library if one is not specified. Since there was an issue with your config file, it is reasonable to presume that it did not specify your database library as well.

Probably the best approach is to use FTP and remove the configuration file (zp-data/zenphoto.cfg) and, run setup and fill in the correct database credentials. You might find a backup configuration file, but it is also possible that it was copied from the corrupt one.

The zenphoto.cfg file seems to be badly damaged, with lines going like this $conf['db_software'] = "$conf['db_software'] = "$conf['db_software'] = "$conf['db_software'] = "$conf['db_software'] = "$conf['db_software'] = "$conf['db_software'] = "$conf['db_software'] = "$conf['db_software'] = "$conf['db_software'] = "$conf['db_software'] = "$conf['db_software'] = "$conf['db_software'] = "$conf['db_software'] = "$conf['db_software'] = "$conf['db_software'] = "$conf['db_software'] = "$conf['db_software'] = forever!

Is it supposed to be a copy of zenphoto_cfg.txt with the right values in places? If that's the case, I could reconstruct a new one; because I think this one is corrupted beyond repair!

Sorry sbillard, I didn't see your answer at first (once more time, forgot to subscribe to the feed...)

I didn't realise the deletion of zenphoto.cfg was enough to launch a new setup; anyway, I have now an 500 Internal Server Error and as I've not modified anything, this might indicate some problem on the server side.
I will try again in a few yours (time to sleep, here...) and report afterwards.

Thanks for helping.

In fact, the server is OK, as I have access to my root page and to other parts of the website. The 500 error seems to be only on the Zenphoto gallery.

Anything else has changed in the new release? According to my provider documentation, 500 errors are often related to htaccess files.

The .htaccess file is greatly changed (simplified) but old files should work, they just have redundant redirects. You should be able to remove the .htaccess file as well so long as you have not modified it.

500 errors are also the result of file permission errors on php script files, so you should also check that.

Thanks for your answer.
As I'll be away soon for a short vacation, I just restored the whole system from a couple of days before for now, in order to have a working gallery.
And I will see in details how to upgrade properly when I'll be back.