Similar problem with my 2005 Ducato 2.3 SWB.
For as long as I can remember, the parking light function would not work: the key would not physically turn back to that position when I pressed the button in.
Recently it actually worked and I was congratulating myself on having an extra part of the van now working, rather than bits ceasing to work, which is normally the case!
Next time I used the parking lights, all was well until I tried to start the engine: the key would not turn far enough to even get the ignition on and no amount of manipulation/wiggling would change this.
I have now temporarily resolved the issue by unclipping the ignition switch from the back of the barrel and operating it by hand. With the switch removed, the key will turn normally in the barrel through all the positions.
Because I have an immobiliser chip in the key, there is no danger of driving off with the lock still engaged, as the engine will not start without the key being placed in the barrel.
Does this sound like the same fault + cure mentioned in the previous post?