I noticed that the monthly archives on my blog were not working. I first thought it was a problem with the .htaccess file but it was correct.

I also tried resetting the permalinks option in WordPress to default an back but that did help as well.

After some digging I was able to fix it by selecting an option in the Robots Meta plugin I use:

ArchiveSettings

I selected the Disable the date-based archives option, saved settings and de-selected it again. I am not sure what it changes (the .htaccess file was not changed) nor did I check the code to see what it does but at least it’s working again!