leocg: Thank you for the hints.
Laptop is protected by Kaspersky Endpoint Security with weekly complete check for malware - check was of course performed in addition when problems occured - no malware. No unusual additional network traffic. No other software performs in an unusual way including mail servers and clients (The Bat).
Pop-Up and links: Opening an definitively unsuspicious site as www.t-online.de (Address and network routing checked for malrouting and phishing) opens additional tabs with advertising sites as well as advertising pop ups, never seen this for years up to Opera 23.
Treating problems with software is always performed with admin rights.
Choosing MS IE as standard browser works well and IE as well as Windows remembers the settings.
With Opera 12.17 and Opera 24 both installed (of course in different directories) control panel has difficulties distinguishing these browsers. No problem with Opera 23.
So, apart from the issue with automatic updates without permission by admin / user, which you did not mention in your comment, the problem seems (on my pooint of view) to be Opera 24.
As I mentioned above, I was quite happy with Opera for years, with a little loss since Opera 21, but I do not want to spent either my time nor the time of my employees repairing software bugs. So, if there is no change within a reasonable time, Opera will not be the standard browser for our company even longer.