@ftwig 

I can imagine there may be spring loaded contacts inside the slot that are designed in a way that they will only get deflected if something is being inserted from the outside, but not in the other direction. Now that those springs have rebound into the cutout of the SIM card adapter, they may be blocking it from coming back out of the slot.

If that was the case I would try to find some sheet of thin yet sturdy enough material, cut a long strip out of it that has the same width like a Micro SIM card and then slide it carefully in the slit between the SIM card adapter and the spring loaded contacts of the SIM card slot. Once the strip is fully inserted try to push the SIM card adapter in and see if it now slides out once it becomes unlatched. 

As for the thin yet sturdy strip, you could try to cut it from some blister-packaging that consumer goods are often packaged with, which is usually easy to find in every household.