I figured the site had been hacked. Almost every day, there are totally nonsensical "keto" spam posts and every day, they get removed and the poster(s) banned. I thought this may have been some kind of revenge.
It is nothing more than an administrative oversight.
In order to thawt identity thieves & other cyber criminals the site uses a method to verify that the site is real called a Secure Socket Layer as signified by the "S" after http in the address.
These are purchased on a short term basis from a third party.
Usually the hosting company renews these automatically, apparently Lawn Site does it themselves and when the certificate expires on Friday after the office is closed this happens.
Quite normal just a little frustrating.
The purpose of it is to prevent a naughty person making a spoof forum then hyjacking the web address so every one goes to the naughty site.
It is fairly common and the biggest offenders are porn & fake medical drug sites so you sit at your computer and attempt to connect to the forum , even type in the correct address manually yet every time you end up at the rouge site.
So basically your computer goes here then it asks a third site "is this the real Lawn Mower Forum ?"
If the certificates are current it says "Yes" and the best mower forum on the web loads on your screen.
If it is NO then you get the message that we all saw.
It is very much the same as internet banking that sends your phone an SMS with a security number that you have to type in before it will make the transaction.
Search "SSL" or "https" and there are lots of sites with graphs & diagrams to explain exactly how it works.
Not all that important here because there is very limited benefit from hacking into the site but it can be used to get viewers to download stuff like key trackers & other mal wear onto their computers.