I had to look up what the Dutch climate is like and I find it quite similar to our climate here in my homestate of New Jersey; US Mid Atlantic East Coast. I personally believe your troubles are created because of the bike being kept outside.
I just took a peek into my Haibike's Yamaha Original Instructions booklet that came with the bike. It's not as in-depth as the X-94 Service Manual, but from it, I quote:
Diagnosis Mode:
The ebike systems are equipped with a diagnosis mode. When you turn on the power, if there is a fault in the ebike systems, the systems will notify you of the fault by flashing the assist mode indicator and the battery capacity indicator, alternatively and displaying Er in the speedometer......
I may be wrong and please correct me if I am, but your bike is in a constant Diagnosis Mode; with power level selection and actual power assist, not working at this time. You speak of replacing the LCD display and the holder it fits into, but you do not mention replacement of the power switch which controls the LCD display & functions as well as powering up or powering off the ebike drive system. Was this switch replaced at the time of the replacement of the LCD and holder??
If this were my ebike, I'd get it inside. At a minimum, get it inside where it is warm and let it dry out for a day or two. Again, if my Yamaha drive, I'd swing the motor down using the bottom non-drive side engine mount bolt as the pivot. Note the picture of my PW..... Note the white connector plugs? With the battery off the bike, I would carefully look at every plug, every wire running off each plug for any abnormality. I would disconnect one plug at a time, clean it with electrical contact cleaner, visually inspect the metal posts for any abnormality. Then I'd lightly apply a silicone dielectric grease inside the plug connectors to help keep out moisture; finally reconnecting it and moving onto the next plug. Do note on your bike that the wires are tightly compacted together. When the motor is tilted back up into place, you certainly do not want to pinch any wire between the motor and that metal motor mount casing it bolts to!!! Finally, retorque the motor mount bolts to the spec listed on the bolt washer.
It is still possible you have water intrusion/dampness into the LCD holder or the main power switch. Ensure the wire runs from these items, going into your frame are not chafed to bare wire. Give your bike some time in a warm ambient environment and see if the system comes on with no flashing diagnosis mode, before any other work is accomplished on your part. Let us know what you find and good luck!
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