On April 24th the ssl certificate of cloudfoundry is renewed.

Directly after that I got errors on the certificate:

**ERROR** Unable to install Libunwind: Get https://buildpacks.cloudfoundry.org/dependencies/manual-binaries/dotnet/libunwind-1.2.1-linux-x64-80af276a.tgz: 
x509: certificate has expired or is not yet valid

Workaround

My workaround was to create an offline buildpack.

Fix

On their Slack channel I got the tip to look at the time setting on Cloud Foundry.
https://starkandwayne.com/blog/bosh-director-time-drift-2/

bosh ssh -d cf -c date

Listing confirms time drift: CF thinks it is still April 17th. Fixed it by setting the date to April 26th:

bosh ssh -d cf -c 'sudo date --set="2019-04-26 08:37:00.000"'

Now the buildpacks work again.