Sounds like broken browser sniffing

No. The actual reason is missing public_domains.dat in 12.16+ (x64/Windows) distribution setup. That's why VK.com, flashscore.com, twitch.tv (and i guess many other similar web-sites) doesn't work correctly. It wasn't fixed in 12.17 x64/Win! Seems that Opera Software don't care anymore about quality control for Presto versions. But the cookies support and stability was broken in 12.15+ (both x86/x64, any OS) regardless of public_domains.dat.