I just had what sounds like the exact same problem with Opera 12.15, but I found a workaround. Flash worked the last time I tried it and then today it didn't work and I was getting messages that I needed to install flash, even after I just did several re-installs. But I fixed the problem by going to a recent file backup of Opera and identifying any different files (I used BeyondCompare file comparison program), and I found some files in the 'defaults' folder (C:\Program Files\Opera\defaults) and also a couple of files in the 'profile' folder I think, I forget, but it was only about 6 files altogether that were different between the existing Opera and the very recent backup. Since I knew that Opera hadn't changed much, if at all, I copied the old files back into the Opera folders, and Presto! Problem solved. Flash is now working. So I don't know exactly what caused the problem, but at least I was able to overcome it. The only thing I can think of that might have caused the problem is a Windows XP Update that I ran last night, but that doesn't sound very likely.