Sorry to hear about the 404s on the Older Posts link in the Lynx theme Andy, we will do everything we can to help!
This behavior you described sounds like it could be related to your website’s Permalinks settings, specifically the Category Base option. Navigate to Settings > Permalinks in your WordPress dashboard and check if there is a “.” (period character) in the Category Base field. If there is, remove that character and Save Changes on the Permalinks page.
Sometimes installing an Inspiration creates that configuration in the Category Base and it can interfere with dynamic page navigation (category pages or tag pages) when combined with the /%category%/%postname%/ Permalink structure.
Hopefully that fixes the problem you are seeing with the Older Posts link on your Aquarium Observations category page and please let us know if there is anything else that we can do to help!