The security certificate expired on August 31. These days, updating certificates is normally handled automatically on the web host. One of the developers probably just needs to click a few buttons to get a new certificate issued.
An expired certificate isn't normally a security risk, at least not for looking at something like the OpenLP manual (your bank is another story!). I reckon you can click the 'visit anyway' (or whatever it's called) button in your web browser to view the manual.
