FAC 5.1.2 and FSSO on FGT fails with bad password

I'm failing to establish a connection from the FAC 5.1.2 to Fortigate (5.6.2 and 5.4.5) FSSO

Debug on both is showing it as incorrect password but I have ensured password is correct both ends..

If I untick the 'Enable authentication' on the FAC both Fortigates connect immediately.

This was a new deployment from a fresh 5.1.2 HV Template. Is anyone able to replicate this?