When booting a system you see the above error. If you lookup the iPXE error URL (http://ipxe.org/0216e48f) you will see that it's caused by the 2PXE Certificates expiring and has a simple fix.

DeleteĀ  the files shown below (you do not need to delete any other files such as cross-signing certs etc). Then restart the 2PXE service. The files will be recreated and the issue should be resolved. This will be fixed in the next release.