The expiring Let’s Encrypt DST Root CA X3 can cause problems on old servers:
root@server:/etc# wget https://www.example.com/monitoring --2021-09-30 21:34:39-- https://www.example.com/monitoring Resolving www.example.com (www.example.com)... 93.184.216.34 Connecting to www.example.com (www.example.com)|93.184.216.34|:443... connected. ERROR: The certificate of `www.example.com' is not trusted.
Fortunately there is an easy workaround to ensure the expired chain is not checked.