Finally, I can give a feedback on this. Replaced both O2 sensors, upstream and downstream. Also reset the fuel trim history (actually, workshop has done it).
At first the motor ran really really bad, but a after half hour or so it leveled out. Now after about 10 hours or so operating, the motor runs really smooth, the slightly rough idle is gone. Also, power returned.
This is the picture of the downstream sensor we pulled out. Looks like it didn't measure anything, but it didn't throw a code either.