BIGIP LTM failed to start with config after upgrade

Did you re-activate your license before you upgraded? If not, and assuming you are in the functional 11.2.1 volume, run "tmsh show sys license | grep Service". The check date should be past 2013-12-05 (http://support.f5.com/kb/en-us/solutions/public/7000/700/sol7727.html) - Only reactivate on a standby system as license reactivation may reload the configuration.

Once that is done on the 11.2.1 volume, copy your config from the v11.2.1 volume to the v11.5.1 volume using cpcfg http://support.f5.com/kb/en-us/solutions/public/14000/700/sol14724.html