And what does any of that have to do with the Master Password?
The error you're complaining about is a bug in Chromium which Opera unfortunately inherited. But a Master Password was something else present in older Presto-based versions that prevented guests from using your saved passwords at all if they didn't know the master password, and this bug in showing passwords does not prevent you from using them.
As the thread is misnamed, there are so many threads about the actual issue, and the issue is already fixed in Beta, this thread is closed.
There are many posts refering to Flash problems.
Search them and try its workaround options, maybe fix your case.
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@magicgalaxy First of all you should read the forum rules as the title of this topic is invalid and also there are already topics opened about the changes on how Flash will work from now on Opera and other Chromium based browsers.