Techland has explained that patch 1.2.1 of Dying Light wasn’t meant to block fan mods, and they are working on restoring functionality.
The patch was meant to block cheaters, and obviously, they didn’t see where their measures would stop modders.
This is Techland’s statement to the modding community:
At Techland, we have always supported the mod community, and loved seeing how our own game can be changed by the players. A big part of the original Dead Island’s success was the passion and creativity of mod-makers from our community. We want the same for Dying Light. For quite some time, we have been working, and still are, on giving modders all the power we can. We will keep you updated!
As for DMCA notices, Techland says they’re unaware why those came out but they’re resolving that issue as well.