I want to add a solution in case you do not use the docker image, to save you some time that I already spent in figuring this out.
On the WOPI client (the external machine, where I installed loolwsd), I had to adjust the file /etc/loolwsd/loolwsd.xml to include a host entry for the domain or IP adress of your cloud installation, e.g.
<host desc="Allow cloud domain or IP" allow="true">cloud.example.com</host>
After this, restart loolwsd: # systemctl restart loolwsd
Now, it is working for me.