It seems that you're using an outdated browser. Some things may not work as they should (or don't work at all).
We suggest you upgrade newer and better browser like: Chrome, Firefox, Internet Explorer or Opera

So, I had that friend who started playing a bit ago and me, who started playing earlier. I'm a bit more advanced on the main quest than him. Can we play co-op on, say, his game? Which are the blocks on coop mode?

Also, after having to redownload the wole game because I couldn't accept challenges, now the game crashes every single time I leave one (i.e. getting killed). Any idea on this topic?
don't know about crashes, but I know about the multiplayer.

Lets say you are 40% complete and friend is 30% complete.

You host, it will tell him the story is further along than his save, and he might miss part of the story.

He hosts, you get a message that the game is further back than your saves, and your progess will not advance until you pass your current save point.

Its best for the person with lowest progression to host until they caught up.
It seems the coop had a problem with configurations and skills (I was playing on Hard, he on Normal, but he switched and I couldn't find him, blah blah). Now its working fine.

Challenges are still crashing my game. Both in SP and Coop.
Well, in case someone stumbles upon this thread with the same crashing problems... (and given support did nothing to help)

It seems the problem was with the Dying Light Manager all along, and reinstalling the game did nothing, because the files that it changes are in the game user's directory (where saves are located, somewhere in /My Documents/My Games/Whatever/).

Deleting the config files (make a backup of the saves first!) made everything work again. I had to re-bind all the keys, but at least I can go back to smashing zombies and jumping from roofs!