Like many of the folks out there, the other day I found out that my monthly archive links are NOT working. When I clicked on any of the past months, they went back to my homepage.
Then I read somewhere this problem is caused by “Robots Meta” plugin by Joost de Valk. Anyway, this is what you need to do:
1. Log into WordPress admin.
2. Go to Plugins and look for “Robots Meta”.
3. Click on Settings.
4. Now scroll all the way down until you see "Archive Setting" section.
5. Remove the tick in the box in front of the text: “Disable the date-based archives” by clicking on the tick sign as shown below:
6. Then scroll down and click Save Robots Meta Settings button.
Now go back and check the monthly links on your archive page.