I also checked:
- Upnp, set up manual portforwarding
- Followed every recommendation, uninstall, reinstall, update, removed Preferences.xml …
- External access works with dyndns and external IP, there’s an issue with the internal access though, cannot access by internal IP nor by bonjour name (which is very puzzling)
- All other applications and services seem to run just fine
- Happened some days ago, I did not update or install anything (maybe there was an automatic update of the synology server, not sure)
I would guess that it’s either an issue with Synology and they messed something up - or you changed something on the Plex side of things? Or we’re all using AVM FritzBoxes and they messed something up?