I finally got it and will share my “technique”.
After pushing and pulling here and there and loosening things a little already, I carefully pushed a knife’s blade 
{ see note}* into the crack between the display module and the main body. I did this several times all the way around the phone. I also moved the knife a little to widen the crack in order to loosen the grip of the plastic studs. Make sure not to move too far so as not to break the studs and don’t cut into the display material. I have no idea whether this step was helpful and am aware of the risk, but I wanted to include it here anyway.
Then I held the phone between my legs the way shown in the picture. Do this with relatively sweaty hands 
in order to maximize friction. Your left hand’s finger tips should rest against the plastic edge in front of the SIM card slots and push them upwards (in the direction of the camera). This will hurt a little due to the fact that the plastic edge is sharp. Also be careful not to let your fingers slip and push the SIM card slots themselves, which could damage them. This push, together with the grip of your left hand on the rest of the main body (pushing upward) and your sweaty right hand pushing the display downward (into the direction of the micro USB port) should do the trick.
{*} [note moderator Lidwien] Please don’t use a knife, it’s to sharp. You could damage the phone and yourself. Use a guitar pick or something simelar instead.
