PXE Error after booting - The certificate associated with this media has expired

Error: the certificate associated with this media has expired. New
Media with a valid certificate will need to be created.

If your boot media cert's have not expired then this is easy to fix as it will just be the PXE point that has expired.

1. Go to Site Database --> Site Management --> --> Site Settings --> Site Systems and choose the server where the PXE Service Point is located.

2.In the right pane, right click on the ConfigMgr PXE service point and choose Properties.

3. Click on the Database tab and locate the Create self-signed PXE certificate option.

4. Under Create self-signed PXE certificate, set the Set expiration date option to some time in the future.
5. Click OK.
6. Go to Site Database --> Site Management --> --> Site Settings --> Certificates --> PXE and verify that there is now a valid non-expired non-blocked Certificate.

This should fix up your issue.. However if your boot media Cert's have expired then you need to rebuild your boot images with new Cert's