Since yesterday I'm playing around with the organ of a local church, I've the feeling that the keys of the organ, which can be pressed down very easily, were ruining my piano technique/touch, is this quite possible or just my subjective feeling?

The organ does require a different keyboard touch - but it depends heavily on the type of organ, the traction mechanism, and on whether you use couplings. Personally I have found that playing organ is not bad for my piano playing. Quite the contrary, it seems.

Just accept that they are different instruments, calling for different music and different playing. Worst thing you can do is play the organ as a pianist, or vice versa. If you are serious about organ playing, get a teacher asap before you learn it all wrong.

The organ does require a different keyboard touch - but it depends heavily on the type of organ, the traction mechanism, and on whether you use couplings. Personally I have found that playing organ is not bad for my piano playing. Quite the contrary, it seems.

Just accept that they are different instruments, calling for different music and different playing. Worst thing you can do is play the organ as a pianist, or vice versa. If you are serious about organ playing, get a teacher asap before you learn it all wrong. It seems unlikely that one day of fooling around on the organ is already ruining your piano technique

An electrical organ keyboard (some pipe organs have these) is considered by most people bad for your technique, as a digital piano is for your piano technique.

What organ is it BTW ? Do you have free access to it ?

oh, It's an acoustic organ with pipes, I can practice from 8 to 12 am during the week, at the weekend I don't know it, I just played some bach 2 and 3 part inventions until now, and i tried to slaughter around with the first piece of bachs orgelbüchlein, but creating a legato with the pedals seem to be really sick;-) good that no one else was in the church^^

The inventions sound good on organ. The Orgelbüchlein is the way to go though.
You should not attampt to play legato with the pedals, at least not in Baroque music. There needs to be some air around the pedal notes, so generally play toes-only. Remember that sounds carry in a church, and what sounds right to you may sound muddy to a listener back in the nave. The more transparant you play, the better.

Thanks a lot for your kind support! I think I'll search for a teacher.