after performing the basic configuration I had a look at the installed packages and performed an update. Since then, I am no longer able to access LUCI because of a 403 Forbidden when calling url "http://192.168.200.1/cgi-bin/luci/admin/system/flash"
You probably ran out of flash storage space if you are using a standard embedded router.
You can boot into failsafe mode and the reset the router to defaults using the firstboot command.
Upgrading packages (via the CLI opkg upgrade command or the LuCI Upgrade... button) can result in major problems. It is generally highly discouraged, unless you know what you are doing or if there is specific instruction to do so.