Solved
This also drove me crazy looking for a solution.
Open preferences for the Mail app. (Click dropdown menu Mail|Preferences)
Then change Default Mail Reader to Mail.
Restart the browser.
Now when you click on a mailto link, the mac mail app will open.
Same problem as what everybody is experiencing. Any fixes? Very Frustrating! Screen Shot 2021-01-01 at 6.32.09 PM.png
Screen Shot 2021-01-01 at 6.30.19 PM.png Screen Shot 2021-01-01 at 6.28.11 PM.png
Well, three months later, and a new update, so I tried again, this time with Version 73. It opened nicely but within two or three minutes it froze, Activity Monitor showed once again, it "was not responding" and had run up 100% of my CPU. What is the deal? Why can't Opera work? I'm on an iMac with 64 Gigs of RAM, running 10.14.6, and Opera will not work at all!
I'm still on Mojave, tried to use the latest update of Opera, a few windows opened, all Opera ones, then within a couple minutes it all froze, ran up 100% of CPU usage, and I had to force quit it to close. This has what I've been dealing with for the past few upgraded versions. No matter what I do, I can't get it to just work! No problems like this with any other browser I use, just Opera. So frustrating, as I would like to use it, but it's become completely dsyfunctional.