Intereseting, must be some routing on the server or firewall rules on the ER-X that messes this up since the other VPN client can reach the ER-X without problems.
Try to compare the routing tables between the OpenVPN server and the Mac connected over VPN.
I'm not sure what to look for, but I would probably tcpdump on both ends to see if my traffic was passing or not. As well as trying other protocols (like SSH or similar).
https://help.ubnt.com/hc/en-us/articles/204962304-EdgeMAX-Capture-packets-on-the-router
The route 0.0.0.0/24 is a bit strange to me... Can't really tell why you are getting that route...