We and some customers experience SSL issues following the change of Let's Encrypt root certificate. This can for example happen if your application tries to connect to external web services or it is using divio SSO.
Depending on the base image you are using, the root certificate bundle might be too old and relies on a root certificate for Let's Encrypt that expired today: https://letsencrypt.org/docs/dst-root-ca-x3-expiration-september-2021/
If you are using python, you might have to upgrade the version of requests and/or certifi in your application in order for it to recognize the Let's Encrypt certificates used by control.divio.com and other domains again.
Our GIT and SSH services are currently also impacted by this and we are working on a resolution.