Is this contrail from an airplane or a rocket?
I am pretty sure it is an aircraft. 

When aircraft fly straight towards you they can appear like rockets. The contrail is distorted by winds in the upper airspace. Yesterday there were a lot of cirrus fibratus and cirrus uncinus in the upper atmosphere (at least above Frankfurt, and we are in the same weather system at the moment), and when aircraft fly through them the contrails typically wobble like in your picture. 



Now, which aircraft was it? 

The aircraft visible on the picture is not flying in sunlight anymore, but most of its contrail still is. It also just made a turn. 

On the eleventh of April, sunset in Leiden was at 20:33 local time (18:33 UTC), setting in a bearing of 285 degrees. 

Flightradar24 show this DHL Airbus A330 descending from FL 390. I would estimate that this picture was taken at 20:36 local time, perhaps a bit later. The turn is clearly visible on the picture and on FR24. The contrail matches that of a large twin engine. (my first guess was a 777, an A330 has a similar contrail. It is definitely not a 747).



A comparison picture created by ymb1 shows the shadow/lighted parts of the contrail highlighted, contrasted with the terminator on the map.