WebUI won't open

I installed HA as a VirtualMachine in proxmox environment.
Actual installed version is HomeAssistant OS 13.1.

I messed up my config: played around with the network settings and the letsencrypt certification and now the WebUI won’t show up in my browser. The only message is
Unable to connect to Home Assistant. Retrying in […] seconds…”.

The Observer URL http://homeassistant.local:4357 works:
Supervisor: Connected
Supported: Supported
Healthy: Healthy”.

Is it possible to change my network configuration by editing config files on the HomeAssistant OS? And if yes, which file(s) and which entries? I can reach the HomeAssistant console via my proxmox servers WebUI. (Is it also possible to ssh into HomeAssistant OS?)