Hello everyone! New update has arrived.
Сo-op mode:

• Fixed: unavailability to place turrets in single mode just after playing coop.
• Fixed: no damage from drones.
• Fixed: incorrect behaviour of drones on co-op mode.

Look and feel:

• Smooth fade in and fade out of menus.
• Smooth fade in/out of interface elements.
• No constant hotkeys on interface. Now shows up only on mouse over.

If you still have problems with co-op connection, please read this:
http://steamcommunity.com/games/264300/announcements/detail/239017085846677708