I upgrade from beta 1 to 2 and factory reset my router RT-AX86U. Unfortunately the issue with the le certificate remains. The GUI remains in authorizing state and my (partial) log says:
Dec 14 20:40:37 kernel: Lets finalize the order, Le_OrderFinalize:
https://acme-v02.api.letsencrypt.org/acme/finalize/106348865/6742417819
Dec 14 20:40:39 kernel: [Mon Dec 14 20:40:39 UTC 2020]
Dec 14 20:40:39 kernel: Download cert, Le_LinkCert:
https://acme-v02.api.letsencrypt.org/acme/cert/04dd764c7cc83f38f1c358f41c6196300f34
Dec 14 20:40:40 kernel: [Mon Dec 14 20:40:40 UTC 2020]
Dec 14 20:40:40 kernel: Cert success.
Dec 14 20:40:40 kernel: cat: write error: Invalid argument
Dec 14 20:40:40 kernel: [Mon Dec 14 20:40:40 UTC 2020]
Dec 14 20:40:40 kernel: Your cert is in /jffs/.le/xxxxxxxxxx.asuscomm.com/xxxxxxxxxx.asuscomm.com.cer
Dec 14 20:40:40 kernel: [Mon Dec 14 20:40:40 UTC 2020]
Dec 14 20:40:40 kernel: Your cert key is in /jffs/.le/xxxxxxxxxx.asuscomm.com/xxxxxxxxx.asuscomm.com.key
Dec 14 20:40:40 kernel: [Mon Dec 14 20:40:40 UTC 2020]
Dec 14 20:40:40 kernel: The intermediate CA cert is in /jffs/.le/xxxxxxxxxx.asuscomm.com/ca.cer
Dec 14 20:40:40 kernel: [Mon Dec 14 20:40:40 UTC 2020]
Dec 14 20:40:40 kernel: And the full chain certs is there: /jffs/.le/xxxxxxxxxxx.asuscomm.com/fullchain.cer
Dec 14 20:40:40 kernel: [Mon Dec 14 20:40:40 UTC 2020]
Dec 14 20:40:40 kernel: Installing key to:/jffs/.le/xxxxxxxxxx.asuscomm.com/domain.key
Dec 14 20:40:40 kernel: [Mon Dec 14 20:40:40 UTC 2020]
Dec 14 20:40:40 kernel: Installing full chain to:/jffs/.le/xxxxxxxxx.asuscomm.com/fullchain.pem
Dec 14 20:40:41 kernel: CONSOLE: 047399.339 wl1.0: wlc_send_bar: for e0:d4:e8:99:54:80 seq 0x1 tid 4
Dec 14 19:41:00 rc_service: service 7744:notify_rc restart_letsencrypt
Dec 14 20:41:02 kernel: [Mon Dec 14 20:41:02 UTC 2020]
Dec 14 20:41:02 kernel: Domains not changed.
Dec 14 20:41:02 kernel: [Mon Dec 14 20:41:02 UTC 2020]
Dec 14 20:41:02 kernel: Skip, Next renewal time is: Fri Feb 12 19:40:40 UTC 2021
Dec 14 20:41:02 kernel: [Mon Dec 14 20:41:02 UTC 2020]
Dec 14 20:41:02 kernel: Add '--force' to force to renew.
It seems that the certificate gets successfully installed but is not shown as such on the GUI. Also the certificate isn't working since WAN access towards the GUI gives an NET::ERR_CERT_AUTHORITY_INVALID error.