Ran into the exact same wall tonight trying to pull a manual off their site. The warning pops because the certificate proving the site is who it says it is ran out, not because anything is actually wrong with your computer or the files themselves. It's an identity check that lapsed, nothing more sinister than that.
I clicked through just to see, and the site itself loads fine once you're past it. Pages, links, everything was behaving normally. I didn't grab the slicer update myself, but if the download is hosted on the same domain it'll probably work the same way. If you want to be extra careful, wait til the cert is fixed rather than clicking through, since that's the cleaner option, but plenty of people do click through on a known site when it's clearly just an expired renewal.
As for how long these things drag, usually not long at all. Once whoever runs the site notices, renewing is a few minutes of work and the fix shows up right away. The delay is almost always just someone not spotting it, often because it lapsed right before a weekend or late at night. I've seen this happen on smaller company sites a few times and it's typically sorted within a day, sometimes two if nobody's watching the inbox. Don't touch your browser security settings to get around it. That's overkill for grabbing one file and leaves you worse off afterwards. If you're in a hurry, try their support or wiki subdomain, sometimes those run on different certificates and one of them may still be valid. Or just check back tomorrow morning, odds are good it'll be sorted by then.
One thing I did notice is that some people are reporting it loads fine from their end, so it might depend on which server or region you're hitting. Worth a retry later regardless.