Portfolio Issue – 404 – Page not Found

Having an issue where when you click on the portfolio item it brings up a “404 – Page not Found”, however when you go back to the portfolio main page and hit the same item then it goes to the actual item.

Please visit my portfolio section to see what I mean.
https://www.billtownwebdesign.com/portfolio/

Was working fine prior to this last update. As always I appreciate all past support and look forward to your help with this issue.

Thanks
Dean