Hi Nathan,
I had a play - a tmsh load sys verify and saw this:
01070726:3: ClientSSL Profile /Common/clientssl-insecure-compatible in partition Common cannot reference ClientSSL CertKeyChain Entry /Common/clientssl-insecure-compatible default_default in partition Nova
Unexpected Error: Validating configuration process failed.
Looks like someone (not me, I promise!!) has modified the default clientssl profile with a (now non-existent) SSL cert / key pairing which prevented the config from loading. Not sure why it loaded into 11.4.1 and not 11.6.0. I edited bigip.conf to remove this section and it loaded successfully!
Cheers