Yeah same error for me in Opera. Launched in Firefox, no issue.
What version of Opera? Version:68.0.3618.125
Is it regular Opera or Opera GX? Regular
What version of Windows? Windows 10 Pro 64 bit
Microsoft Windows [Version 10.0.19041.264]
Is that version an "N" edition of Windows? If so, you might need to install the Windows Media Feature Pack for your OS. No
At https://html5test.com/, does it say "Yes" for h.264 support and "Yes" for AAC support? Yes and yes
At https://bitmovin.com/demos/drm, does it say "Detected, using Widevine" above the video and does the video play? Yes
For MSE below the video, does it have "video/mp4; codecs="avc1.42c00d" checked? Yes
In the EME section, does is "Widevine" checked? Yes
Did you try installing https://addons.opera.com/en/extensions/details/user-agent-switcher-5/ and setting the site to mask as "Chrome for Windows" Yes, No Change
(including editing the "Chrome for Windows" string to be Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/83.0.4103.61 Safari/537.36)? This here, worked!
Did you try going to the URL opera://flags/#disable-accelerated-video-decode, disabling the flag and restarting Opera?
Did you try going to the URL opera://settings/system, disabling hardware acceleration and restarting Opera?