Greetings JanEgeland!
Thank you for reaching out to us.
In order to attempt to solve your issue, if your friend has a better performing PC, please try to locate the saved file for your Co-op game - which you can find in your folder here Steam\steamapps\common\Conan Exiles\ConanSandbox\Saved - then share it with your friend.
Once shared, have your friend run it and see if the infinite loading/freeze stops occuring 
Hopefully this fixes your issue!