as you can see on this picture the washer has a little bit sticking out on the inside.
the washer on my headset has it as well but because of it I can not get that thing
on my fork! whats that? how is it supposed to work? the headset fits fine
so its not a 1" - 1" 1/8 mixup.

most threaded forks have a gutter cut out where that tab goes. you could probably file/grind that tab off.

Does your steerer have a groove cut in it? That tooth is supposed to fit in the groove and keep the upper nut from turning the lower when when making an adjustment. You can file it off, but you may need two wrenches to do a proper adjustment. Since you had one on there before this one might be a little to tall and need some trimming or the old one may have been forced on.

I had that on my headset as well - modern forks come come precut with a groove at the back of them on the threads - since I had to rethread a carbon fork I had since it was too long and couldn't get it on either, I just filed and cut that little protruding part. Sanded it out and it was fine. You could probably get a replacement washer if you didn't want to do that but it worked fine.

ah.. thanks guys. its for this colnago fork and that doesnt have a groove. i have two
wrenches so it shouldnt be a problem. thats the first threaded headset i have installed myself so I am a bit clueless