The issue has been solved by the CumulusMX users forum team.
For the record and assistance of other Windy.com users, I understand it is because Windy uses Let's Encrypt for their SSL certificates, recently (30th Sept 2021) the Let's Encrypt R3 intermediate CA certificate they use to authenticate their client certificates expired.The rPi has not kept up with that change and still has the old intermediate CA cert. (I am no expert!). The steps I followed were as follows:
Updating the mozilla/DST_Root_CA_X3.crt certificate:-
Open a terminal session on the RPi
Type sudo nano /etc/ca-certificates.conf into the terminal session, look through the response and
Then type the exclamation character ! in front of the line: /mozilla/DST_Root_CA_X3.crt
Save the file: Ctrl-X, then hit y for yes
Then update the certificates by typing sudo update-ca-certificates into the terminal session
Reboot your RPi, the Windy.com upload will restart and you should see your data within an hour or so.