Some players (especially from Turkey) have recently reported issues connecting to the online co-op mode.
While this does not affect everyone, a few users have found that the following workaround helped them establish a connection:
Several players reported that changing their Steam download region allowed the online co-op to work for them.
How to try it:
Open Steam Settings
Go to Downloads
Change Download Region
This can sometimes affect how Steam routes the online connection. It may not solve the issue for everyone, but it has already helped some players get co-op running.
If it does not solve the issue for you, please feel free to reach out with details.
Smi.