Full disclosure, my laptop's a disaster. It's Windows 10, pretty much never gets turned off, got Norton 360 on it that's always messing something up, the past few months it's been firing up fans at full speed for every small function, been bugging out in loads of ways just the past week, and I last cleaned out its guts over a year ago, which was to replace its battery
That said, I just got mullvad, and every time I try and open the app, it sends the "disconnected and unsecure" notification, and does nothing else. I disabled Norton's firewall, still nothing, but I don't know if that rules out Norton as the culprit
It's also been acting funky for browsing. Reddit most noticeably never opens properly when running a VPN, same goes for a lot of other sites, the whole reason I got Mullvad was because that was the final straw for Proton, it'd been sucking consistently for a while now anyway. Still, maybe it wasn't a Proton problem and all the other abuses this thing endures are to blame