Archive links problem: No posts found

Hello,

I’ve got a little problem with my wordpress-site on https://thomas-ney.com/.

In my sidebar on the left, I included the following tag to show a monthly archive dropdown menu:
<?php wp_get_archives('format=option'); ?>

But when I chose a month/year, it returns a no-post error, even if there are posts in the selected month.

Same applies to my archive on https://thomas-ney.com/archiv. I use the <?php wp_get_archives('type=monthly&show_post_count=1'); ?> tag there, and the post count shows that there are posts for every month. Anyway, a click on the link returns no posts at all.

I guess it has something to do with the permalink-structure, so I post my settings here:

/archiv/%category%/%postname%.html

Does anybody know a solution for my problem?

Thanks for your help,

Thomas