Goto the URL opera://about and take note of the install location. Then, close Opera.
Open a command prompt and run this command:
"C:\Program Files\Opera\launcher.exe" --ignore-certificate-errors(adjust the path to your launcher.exe)
Then try things.
If that doesn't work, try https://peter.sh/experiments/chromium-command-line-switches/#unsafely-treat-insecure-origin-as-secure, which might be a better option anyway. You can then modify the command in Opera's shortcut to always pass that command-line switch to Opera's launcher.exe.
Or, on the error page, type thisisunsafe as mentioned at https://superuser.com/a/1519667/6488. Don't know if that still works though.