We will look into the posting limits. Sorry about that.
As for the issue on hand, when the WebUI becomes unreachable, does running the command /etc/init.d/uhttpd restart resolve the issue temporarily, or are you still unable to login?
This would help us determine if the issue is related to the WebUI, or should we look somewhere else.
If the aforementioned command does not help, could you try rebooting the OpenVPN service by running the command /etc/init.d/openvpn restart and check if that changes anything?
Update: I think it is working now. Maybe yesterday morning the problem was with the Browser Cache.
During the last almost 40h the problem with the UI-login did not occur again (after updating the modem firmware also).
The problem with all my (Shelly) IoT devices losing the connection to the cloud and do not connect again after a certain time is still not solved. This problem occured after I updated to the latest router firmware last week. I will open another topic for that issue. During the day all device become offline in the cloud, but I can ping them via OpenVPN. Strange.