I found a solution. This response summarizes the advice I got:
After choosing Interface > Default (not Classic) style, the “New” button at the very top of the screen still produces a new post in the block editor. That’s the button I’ve always used. I hardly even noticed the “Add Post” button. But with Default, that button does give me a choice of Block or Classic editor, and those two do deliver the different types of new post that one would expect. Apparently the Default choice is also responsible for restoring Classic Editor as an option when mousing over the title of an existing post in the list of posts. Not sure I understand the wording – Default vs. Classic – but it works.
What if you goto the URL opera://browserjs and disable browser.js for the session (after you do the above clearing of cookies for paypal and clearing of your cache)?
This tip worked. Unfortunately, all the others didn't. Installing a fresh system also works.
The problem occurred after the last Opera update.
@leocg To be honest, it doesnt matter for me, if it happened 1 time or 10 times. It never happened for 10 years since now, nobody have same issue, so I already decided to reinstall complete PC. I dont want to spent more time with analysing and trying. I need to work on my computer without problems.
Thank you for your time and support.
I would suggest not only simply increasing the urls count but also implement some general icon on the sidebar for this purpose to not clutter the sidebar (so when you hover the icon a pop up with the url list will appear )