Help with Er 9 code at start up

Hi, we have a freeze drier in with an Er 9 code on startup - manual states 'pressure switch activated' This is something totally new to my skillset, so I'm hoping someone can point me in the right direction - maybe someone has seen this before? The pressure switch is a Danfoss KP 5, and connects to a compressor. The switch is closed so I assume that this part is OK. There is a Vacuubrand VSP 3000 pressure transducer, that I believes communicates with the controller pcb by a special bus, so I have no way of testing this. My feeling is that the the fault is with the controller but would rather be more sure before I ask the end-user to buy a new controller. BTW, the fault description is 'The freeze drier started making a screeching noise and stopped working (Error code 9 - Pressure switch activated).'.

Equipment: Labogene ScanVac Coolsafe 110 - 4 Pro