Hello,

For some reason, my ZP core (1.4.5.7) automatically restart the setup process (2nd time in a week).

I realize that this could occur when some changes are detected in the php or Mysql server. Well, that's not the case, is a production server used for other applications and no changes have been made at the OS level.

I've entered the MySQL access configuration again and everything seems normal now... but I really want to avoid several hours of gallery downtime in the future if possible.

Some ideias about the reasons that could be triggering the setup?

On ZP 1.4.5.7 only you as the admin should see a not about re-running setup. It should neither block your site nor run automatically.

Why this is triggered is explained here:
http://www.zenphoto.org/news/installation-and-upgrading#re-running-and-re-uploading-setup-files