I'm sure this is something local related to your APS setup. You must be running the Enterprise version of the APS, not Community one. You can use ACS Community however and from your screenshots, I think you don't have problems with ACS on its own.
For troubleshooting the Activiti Enterprise including CORS setup you can use the official support channel, or asking questions for the open source one here: Activiti/Activiti7 - Gitter
The error you see can happen in 2 cases:
1. The server or given URL does not exist or not accessible
2. CORS is not properly configured.
Unfortunately, the information you give is not enough to understand how your network is configured. All I can say is that we use ADF with various network configs and there are no issues reported so far.
I suggest testing everything on a single machine and then moving to the network setup.
And what is the sense for the community to develop a framework for paid software? At the same time, leave a support for the community version of the alfresco content service ... And do not support the community version of Activiti...
I do not in any way express dissatisfaction. I appreciate the work of the community. But usually the community develops versions for the community, and in the case of Alfresco it is not clear why it happened.