on the foreman server I had to run:

puppet cert clean client-certname

on the client

rm -rf /var/lib/puppet/ssl

and

puppet agent -td --server=<foremanservername>

Additional Hints

To fix this, remove the certificate from both the master and the agent and then start a puppet run, which will automatically regenerate a certificate.

On the master:

puppet cert clean <fqdn-hostname>

On the agent:

1a. On most platforms: find /var/lib/puppet/ssl -name <fqdn-hostname>.pem -delete
1b. On Windows: del "\var\lib\puppet\ssl\certs\<fqdn-hostname>.pem" /f
2. puppet agent -t

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