RAX15 NAT Loopback function suddenly stopped working

I have a Synology DSM up and running behind RAX15 router and up until recently I was able to access it with DDNS from local network. Which makes sense, because my router is in the list of routers that support NAT loopback.  

Starting recently though (~month or so) I couldn't access my server from local network with DDNS (only through local IP). 

• The ports are open
• I can access it from external network using the DDNS domain name and port
• I can access it from local network using local IP
   

So my assumption is that it's neither the server issue nor the port forwarding, but specifically NAT loopback function. Any ideas what might have happened? Thanks for your help.