Fiesta mk8 intermittent rear seat belt fault warning

[ajprice]

My dad's Fiesta (2017 Titanium) has started showing an error warning for the rear offside seatbelt on the instrument panel screen (random Google image below of the seat belt screen). It isn't on all the time, he's tried buckling and unbuckling the seat belt but the error comes back. The seat hardly has anyone sitting in it. Is there a fix for this that will stay fixed, is it likely to be a duff sensor or an actual fault in the belt?

[PisstNBroke]

Probably a connection on the harness underneath the seat. Has your dad used seat covers at all for a dog etc?
As I have and if its like the current Focus it can become unclipped easily. I managed to get to it by taking out the isofix covers and a bit of manual persuasion.