After nearly a month away from my site, it told me to update to Wordpress 2.6. I figured, why the hell not (I like living on the edge). I went through the very easy update-process and thought it all went well.
Unfortunately, it didn't. At first I thought everything had broken, all my single pages gave a 404, even the non-blog ones, and all my links to tags and archives stopped working as well! Panic!
Luckily, I found out pretty quickly that I only needed to fill in the 'tags' and 'category' field in the Permalinks section, which solved most of my problems instantly.
The last problem, the archives that didn't work, took pretty long to figure out. I googled trying to find if anyone also had their monthly archive links throw them back to the main page I set the Permalinks to the ugly default, and that didn't work.
By now I started to worry and considered rolling back to 2.5.1 just to get the monthly archives links to work again. Before I did, I did an install in a different directory as a test and that one had working monthly archive links. As the new install did not have any but the default plugins, I started disabling plugins in my default installation until I found the culprit: the robots meta plugin. Once I figured that one out, it didn't take long to find a notice of the creator of the plugin, stating that the update revealed a bug and ladida, problem fixed, just update the plugin and set all your settings again. Which I did.
The plugin has a 'disable date archives' option that got checked. In fact, everything got checked. I unchecked unwanted things, saved, and my monthly archive links work as they should again.