I just checked, and the Mega SSL cert is valid until February 20, 2022, so it's not the certificate that's at fault. I'm not sure what date your computer is set to.
I did have Mega complain at me just now that the transfer quota was exceeded for the Paradise Courtyard item, but I use a VPN and I normally tie-up to a rather popular IP address, so I wasn't too surprised. I got around the issue by switching the VPN over to some Canadian IP address that I knew no one could have possibly downloaded the item from. It downloaded fine from there.
Otherwise, Mega seems just fine. I use a heavily modified version of Chromium running on a virtual machine to download with.