I do want to add one more note to this thread incase someone without so much knowledge reads it. The prime reason for the install signature is because one version of Zenphoto is not exactly like the next.
When you upgrade, setup will migrate your database to be consistent with the current version of Zenphoto. If you do some thing like restoring your database to what existed for a previous version of Zenphoto as apparently SebastianKleine did then you have potentially compromised everything in your installation.
For this reason, and because we run into install scripts that do not do this migration, Zenphoto tests the signature of the install and if it finds anything suspicious suggests a running of Setup to straighten things out.
Perhaps there are occasions where nothing actually got migrated so the re-install would not be necessary, but testing that specifically would certainly be too expensive to perform each time a page gets loaded.
So, bottom line, if you use MySQL to backup and restore your database, be sure you do so with the same version of Zenphoto running. (I would have thought that would be obvious to someone with programming skills, but then....)
Zenphoto does provide a backup/restore facility that is able to deal with many of the migration issues. But it is still recommended that you do your restore on the same version of Zenphoto that created the backup because you will not know if it can handle the migration until the restore.