How do I repair our Polder thermometer display?

Have you tried pressing right on the zebra connector area? Your first instinct was likely correct - poor contact on some of the zebra contacts. It is tough to keep uniform pressure all the way along a long zerba connector. In some plastic cases you can put a shim to keep more pressure on some area that is not making good contact.

I don't think it is possible to successfully re-assemble the liquid crystal sandwich if you take it apart. I would try contact cleaner on the connections or some way of applying pressure.

It's unlikely to be a problem with the LCD glass sandwich and it's more likely a failure of the LCD self-adhesive ribbon which is stuck down with adhesive in this type of assembly.

The only way you can remedy this is to use a brand-new self-adhesive ribbon film and get a perfect alignment, squeegeeing the film down as you go. They're fiddly in my experience with constructing this kit which uses a similar type of connection: https://goughlui.com/2016/10/15/project-cd2003gpsc3610d-fm-receiver-wlcd-alarm-clock-74-108mhz/

Not sure where one can purchase such film though .. so if carefully pressing down on the joint doesn't restore the display for long, then there's not much one can really do. In a pinch, you could try doing something mildly mechanical (e.g. use a bulldog clip and some paper to distribute a clamping force across the contact area) as a kludge.

- Gough

I recall something similar cropping up on the community forum a while back.

You can get anisotropic conductive film tape: 

https://www.3m.co.uk/3M/en_GB/p/d/v000073742/

if you can manage to separate and clean up the existing connection which usually requires a bit of heat eg:

"Bonds made with 3M Anisotropic Condustive Film Adhesive 7303 are repairable
by heating the bond-line to above 100°C (e.g. with a hot plate or rework tool) and
peeling the circuits apart. The bond site then requires cleaning with a solvent, after
which the circuit can be re-bonded using a fresh piece of ACF 7303. Solvents such
as 3M Novec Engineered Fluid HFE-72DA or Acetone may be used."

There was a tutorial showing someone doing this type of repair, which might be worth a search for. 

You probably want to practise a few times on another disposable device first though as it looks a bit tricky.

Adafruit do 'Z axis' anisotropic tape in smaller quantities:

https://www.adafruit.com/product/1656

I also recall there being flexible splices available with the anisotropic tape pre-attached either end, which allow you to re-terminate the ribbon if the end is damaged.    

You can also get T-bar tips for soldering irons that help with such repairs:

https://www.amazon.co.uk/CITALL-Soldering-T-iron-Ribbon-Repair/dp/B07BBMD8DW

in which case a quick reheat followed by some pressure might do the trick, or alternatively help with removal without damage ready for clean-up and new tape.

This is a common problem with these displays.

They almost always degrade with time.

The connections corrode and contact is lost, either in the connector or the device itself.

BD I had to go get the dictionary "anisotropic conductive film tape" where do you get this stuff. Awesome!

I keep it filed next to the elastomeric connectors, which are then collectively filed under 'weird stuff'...