wget: ERROR cannot verify www.important_website certificate
It's not linked to wget but to your SSL certificates trust store (these are used to ensure encryption between client (you) and server via TLS). Either your store was damaged in some way, or the certificate in question is not valid anymore, and you receive this error as the connection can't be guaranteed. From testing, their certificate is still valid, so there are likely some issues with your ssl store.

You can also add this option --no-check-certificate to ignore such errors, which would solve your problems. However, do it at your own risk, as those certificates are there for a reason.