Can' sign on because of SSL key verification

I am unable to log on to my Mega account. I get the following error:

"Our SSL key can’t be verified. You could be affected by a man-in-the-middle attack or your antivirus software could be intercepting your communications and causing this problem. Please disable it and try again. (Issuer: Unknown)"

I have Windows Defender as my only AV. I tried turning it off to no avail. Any suggestions would be most appreciated, thanks.