Hi,
I’m facing the same error with various builds that use to fetch (git) stuff from several servers.
Just guessing (given the dates) if the problem is related with DST Root CA X3 Expiration (September 2021) - Let's Encrypt
I had the same problem in some own computers (xenial and focal) and had to perform an apt update & upgrade to get newer openssl & certificates-ca packages. After that, everything was back to work.
Maybe the travis environments don’t have those packages updated? I’ve tried here rerunning some jobs with xenial, bionic and focal and all them are failing with the same error.
Just a theory, but the date where errors began is a good coincidence.
Ciao 