The issue has been solved by the CumulusMX users forum team.

For the record and assistance of other 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 upload will restart and you should see your data within an hour or so.

The posts from the CumulusMX forum are here and here.