On this day you may face some issues with Splynx or any other software what uses Let’s encrypt certificates. This is because the root certificate used by Let’s Encrypt to sign client certificates will lose its validity on this day (expiry of Intermediate R3 on 2021/09/29 at 19:21:40 GMT – the DST Root CA X3 expires on 2021/09/30 14:01:15 GMT). Clients that only know the old root certificates will not be able to verify Let’s Encrypt server certificates after that.
More info here: source-1, source-2, source-3.
This issue affects Splynx version 3.0 or less (Ubuntu 16.04, 14.04 or Debian 8-9). You may face problems with some add-ons, web-hooks, SMS and email sending or even with a web access to Splynx.
The best solution for this - to schedule the upgrade to Splynx 3.1, to do this please log a support ticket.
As a temporary solution you can run these 2 commands in the server console:
sed -e '/DST_Root_CA_X3.crt/s/^/!/g' -i /etc/ca-certificates.conf update-ca-certificates