Hi Simon,
Are the cert and key both in PEM format? If not, can you convert them using openssl on the BIG-IP. If you need to convert them, you can use openssl to do so: (
Click here).
SOL6549: Converting PKCS certificates to PEM format
https://support.f5.com/kb/en-us/solutions/public/6000/500/sol6549.html?sr=489149
Else, if they are in PEM format but still aren't matched, can you take a look at this post for details on how to verify the cert and key are a valid pair? Reply here if you have any questions.
Certificate and key do not match (
Click here)
Aaron