Addendum:
When I open the lid again, here is what happens in the cases 1) and 2) described above:
1) laptop screen is switched back on in native resolution, external monitor is switched off (no signal) and has to be re-activated in System > Preferences > Display > "laptop off", "external on"
2) the laptop screen remains switched off and the external monitor works fine again in highest resolution
Interesting: behaviors 1) and 2) alternate when I close and open the lid several times in a row (each time re-activating the external screen after behavior 1)). The Power Management setting remain unaffected on "do nothing" during all this.
Addendum:
When I open the lid again, here is what happens in the cases 1) and 2) described above:
1) laptop screen is switched back on in native resolution, external monitor is switched off (no signal) and has to be re-activated in System > Preferences > Display > "laptop off", "external on"
2) the laptop screen remains switched off and the external monitor works fine again in highest resolution
Interesting: behaviors 1) and 2) alternate when I close and open the lid several times in a row (each time re-activating the external screen after behavior 1)). The Power Management setting remain unaffected on "do nothing" during all this.