UJF3042 "HEAD HIGH" Error

[Forty One]

Back again with another question.
When I print onto ACP or similar, It drops the bed all the way to the bottom just as it starts printing then I keep getting this error warning that says "Head High".
I normally only use this for printing braille signs and these are flat sheen, 1.2mm thick panels.
I really dont have any other materials lying around to test it out on to see if its a glare/shiny thing or thickness issue.

Can anyone enlighten me on what could cause this issue?

 

[Smoke_Jaguar]

Not seeing Head High in my service manual, is this a MKII or old MK1? Make sure the sensor's reflector is catching and reflecting the light back to the sensor. Sometimes ink on the retroreflector or the bracket mounting the sensor can get out of whack.

 

[TD Digital]

Do you have something that is a known thickness? Like a 1/4 inch acrylic? Place in each corner and press remote. Deviation should be less than .1 mm in each corner

If it drops all the way the optical sensor is so misaligned it's seeing the table or carriage.