Port 8448 not reachable from outside

What type of hardware are you using: VPS bought online
What YunoHost version are you running: 12.0.11
How are you able to access your server: The webadmin
SSH
Are you in a special context or did you perform specific tweaking on your YunoHost instance ?: no

Describe your issue

Ufter upgrading to Yunohost 12, the problems are getting fewer. One is the diagnosis:
[ERROR] Port 8448 ist von Aussen her nicht erreichbar (Port 8448 is not reachable from outside).
I am on a rented VPS, which does not offer a firewall. the suggested port-forwarding seems to refer to a server in my home behind a router, which is not the case.

This issue prevents Synapse from starting.

What else can I do to find the cause of the issue?
Thank you in advance.

Share relevant logs or error messages

https://paste.yunohost.org/raw/etebinusor

sudo yunohost firewall list
opened_ports:

  • 22
  • 25
  • 53
  • 80
  • 443
  • 587
  • 993
  • 5222
  • 5269
  • 5349
  • 5350
  • 5353
  • 8448

I ran the offered Yunohost updates including Synapse. After that, Synapse is running and port 8448 is reachable.