As mentioned above, I have deactivated all add-ons.
The first couple of sections are visible and readable and clickable, however, about 1/3 down the feed there will be a clear-cut break-off beyond which there will be only white. The loading symbol in the bar will continue to be active and after some painfully long time, like 1-3 minutes, sometimes the rest of the page will appear, sometimes only about another third.
This affects only Spiegel.de. All the other pages load without any problems. Maybe I will try to reinstall Opera again. I have tried before already. Maybe it will work this time.