Certificate problems since Let's Encrypts CA expired a few days ago
Disable "DST Root CA X3" the certificate on your server. Run:
sudo dpkg-reconfigure ca-certificates
On the first screen that prompts "Trust new certificates from certificate authorities?" choose "yes". On the next screen press the down arrow key on your keyboard until you find mozilla/DST_Root_CA_X3.crt, press the space bar to deselect it (the [*] should turn into [ ]) and press Enter.