Thanks a lot Curt! This one finally DID the trick!
Btw, someone has been asking where in "sandboxie.ini" to paste that line. FYI, it worked for me at the end of the "Default Box" section.
Finally, may I ask you, Curt, if your time permits, to please elaborate a little bit what exactly this line does and why it has become necessary for this latest Opera-build? Over there at Opera-development it looks a lot as they would indulge in implementing unsolicited changes with every new build.