"Plus, as noted at the comments, to avoid that hussle, the package's maintainer hosts an arch-compatible repo at repo.herecura.eu."

Thanks. I added that repo but package could not be found.
Anyways my startup problem is gone, whatever was causing it. it appears that installing opera-ffmpeg-codecs fixed it.

Looks like I have solved the problem by installing opera-ffmpeg-codecs. Opera now starts every time.
I was reluctant to install this package from AUR because of 500MB download that ends up in a 3MB package. How ridiculous is that?!!

So I am now happy returnee to Opera! Best browser ever!

After examining the problem little more it looks like that Opera does close, but you cannot restart it again. While watching running tasks I observed that when Opera starts it starts few processes. Now if I kill one process, and it has to be the right guess, Opera will open.

I have connected this behavior with how Opera starts. If I set it to "Continue where I left off" this is when Opera fails to restart. If I choose to start with Set of pages, Opera will start every time but it forgets window size and position.

In both cases pages I chose to open at start are the same.
Any fix for this? Is it a known bug?

I have been using Opera since version 8.0 in windows. For many versions this problem plagued Opera. It does not close, keeps running and you can't start it until you kill all instances using task manager.
I can't believe that this obviously bad code still lingers in Opera, now in Linux.
Could not find anyone else posting about this, can't be just me.
I am on Manjaro Linux 17.02 with Plasma KDE.