A few words about debian and the ppapi version of flash.
Debian, on its main repo, offers a package named "pepperflashplugin-nonfree". That is essentially a script that downloads the right package from adobe, extracts it and places it in all the right places.
However, if you take a look at its bug reports, you will realise that this package is poorly maintained rubbish, as I had said in a similar thread.
Since you are on debian stable, be thankfull that it has been removed from the repo, because it's poor quality does not qualify for the standards of stable, and pray it won't make it to the next stable.
What you can do is install the repo of deb-multimedia.org, which offers ppapi flash in the "flashplayer-chromium" package. This one really contains the plugin from adobe inside (= does not download it) and it is maitained by an excellent and responsible maintainer.
If the above works, please give me some credit by liking this post, because I was the one that suggested to that maintainer to package it ~2 years ago, because I was fed up with that bs on the main repo.
Once you are done, go to opera settings > adnvanced > privacy and security > content settings > flash and enable "allow sites to run flash".
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As said above, the last opera version that did have a mail client built-in was 12.16. It has not been updated 5+ years and today it is as obsolete as internet explorer, because it has many issues with most sites.
I also doubt it it is installable at all, because of its prehistoric dependencies (gstreamer 0.10 and the like).
There also was opera mail, a windows and macos app that never came to linux and never got updated past version 1.0, so that is obsolete too.
There have been several posts about this scenario, both under Windows and Linux. I can confirm that it occurs with Opera 57.0.3098.102 on both Ubuntu Xenial and Ubuntu Bionic. In my case, I suspect it has something to do with encryption of the home directory: accounts with cleartext home directories do not, in my case, exhibit the problem.
I hope this will be a clue for someone to finally diagnose and resolve this issue.
I am really surprised that over 250 folks have read my post and I have received only one set of questions and one possible solution....which unfortunately didn't work......and that's been it. I figured this was a simple fix that I just wasn't smart enough to do, or it was something common that others had already experienced and had worked through a solution. Now I'm not sure if I just have some strange side-effect of using various (approved) apps or whether my computer needs an exorcism. It certainly isn't a game changer....I can always switch to Firefox or Chrome and print to my heart's content, but there is something very wrong when a browser won't let you print at all. Before I uninstall Opera, I guess I will cast this posting out one more time and see if I get any nibbles. Ridgerunner
@sgunhouse It is more hidden but still there. At least on the latest Opera Version for Linux.
After I disabled Turbo, it is now working, but there should be a warning/hint/... to disable turbo if it can't be implemented to redirect to the login page.