EDIT: This issue has been fixed, if you find any similar issues, please contact me immediately. Thank you!
Old Post: As of the latest patch (1.05) there is a bug in co-op where you may lose all equipment. This happens if the host leaves the game whilst in the starting area. The host will not lose equipment, but the client will.
It is possible that this issue may occur in other scenarios too, so to be completely safe please wait for us to patch and fix this issue before playing co-op.
If you have lost any equipment feel free to contact me here on Steam or via my e-mail at email@example.com
We apologize for the inconvenience, LiVam - Fatshark QA