I replied in PM, its something you need to look into yourself, we will not be providing support for this on the forums unfortunately to due the nature of it.
@Wilco care to PM the the details of the work around?
My son was affected by this also starting a day or 2 ago. I can connect fine, he cannot, and we are on the same router (so same external IP).
I tried what I could think of.
Deleting the arma3 mp mission cache thinking it was a bad file.
Verified steam cache (which i didn't think would do much as other servers work).
Cleared DNS cache on his machine.
Changed his DNS servers for the heck of it.
Set him as DMZ to test if it was a port forward issue.
Same issue of connection, with or without the launcher gives him the map loading screen then just a generic "connection failed".