Just updated DSM on my Synology NAS to 7.2.1-69057 Update 5 and noticed that external connection to HA via Synology reverse proxy stopped to work. When trying to access HA from outside I get error 400: Bad Request. Untill update everything was working fine for years, without any issues. No changes in configuration done at the time. Anyone experiencing something similar? Any solution?
Worth to mention; checked several other internal URLs configured for access via reverse proxy and HA is the only one that stopped to work 
OK, got it resolved; for some reason DSM is now not respecting default service order for network cards. I have NAS with 4 LAN ports and 1st one is configured as default . While all reverse proxy request are comning to NAS to this default port on IP 192.168.52.21, reverse proxy is forwarding this request to hassio VM using 4th port, with IP 192.168.52.24. This IP was not configured on my HA as trusted proxy, so connection was refused. Adding this IP to proxy list resolved problem (though it is sort of workaround, as it should be fixed on NAS rather to respect service order).
Interestingly I can’t find any option in DSM to make it working as intended that would be specific to reverse proxy, e.g. using the first LAN port for outgoing traffic. I’ll need yet to verify all other possible services, if by chance this does not affect them negatively as well…