Just want to add to this, as I am experiencing the same symptoms as gromit on an Ultimaker 2+ Extended, the one with the connection style shown in Megatarts image.
Similarly I can do one print each day, then the error persists for some time afterwards.
I soldered jumper wires directly from the connectors to the PT100, and still the error persists.
How likely would it be that the PT100 itself has failed somehow, or the sensor wires from the bed? They do seem to be bending nicely without obvious kinks or strain.
As an additional note, I've only ever seen the error thrown when the bed has fully descended to it's lowest position, not during a print.