Hello,
if your partner receives a 403 FORBIDDEN message, this means that your partner can already reach the AS2-Adapter, so there should not be any errors related to FireWall/Proxy or similar settings...
This error is most likely related to the fact that the AS2-IDs don´t match (and possible causes for this have already been mentioned in previous posts....)
In the screenshots, I could see the CCs and the SAs, but can you also provide a screenshot with the party (where wecan see that the AS2-IDs are assigned correct to the respective Parties whic hare used as Sender/Receiver in your SA ?
Other than this, feel free to check out this blog/doc....
Certificate and Troubleshooting - Guide for Seeburger - AS2 - Adapter
Regards
Stefan