My archive view shows me 54 images uploaded for January. Please see: http://www.synnatschke.com/gallery/?p=archive
But if I click on the link http://www.synnatschke.com/gallery/page/search/archive/2008-01 (link text: "January (54)"), the search delivers 53 matches. There is also a message on top of the page, says: "Total matches for January 2008: [b]53[/b]

Do you have any other archive dates like this (hopefully with fewer images)?

Can you look in your database image table and sort it by date. What do you find for January? Specifically, what are the earliest and latest dates for January? I have not found any anomolies like this on my site, so I think it is probably data dependent.

Which is correct?

I have an entry "zen-logo.jpg" in the table zp_images, which I never created. It has an ID, but everything else is NULL. It also has no date. What's the reason behind, can I delete it?

You can delete it, but it may just come back. It is created when there is no thumb for an album to use as its thumbnail.

Found the reason! One picture had 2 different IDs within the db. I don't know why. I removed one of those entries. Now the search results are correct.

Thanks for the report. That really should never happen--the id is the database key!