@muzka @MichaalHell and others with button problem:
Disclaimer entirely at your own risk, of course.
You can fix the button problem using Caig Deoxit D5.
I have done this successfully on one of my OP-Z, do not be tempted to use any other type of spray, especially WD-40 or contact cleaner which are greasy and will make a mess and probably make it worse.
Deoxit D5 is safe on plastic and leaves no residue, the D5 version is the only product suitable for this particular job.
How to do it.
Remove bottom plate and battery from OP-Z, you will probably want to take a backup first.
Place OP-Z on a table with some kitchen paper or similar under it to prevent mess.
Set the nozzle on the D5 to the lowest setting, and carefully spray a short burst into each button affected, press the button multiple times to ensure that it penetrates the contacts, like 20-30 times and firmly. Repeat for any other buttons.
When done wipe away excess on top and underside and let it sit for a while, probably an hour or so, then re-assemble. You might find that they are all working, or some still do not, if so repeat for any that are not working properly. Mine took 2 attempts to get them all working perfectly.
Edit: A very tiny amount is recommended, you can always add more if needed.
I think the problem happens because dust or oxidation gets inside the small metal domes under each switch, the D5 cleans this out if used in the manner described.
D5 is quite expensive (I paid around £20) but it comes in handy for other things, like cleaning switches and sockets, though do not use it for potentiometers as it removes all of the lubricant, there are other versions of Deoxit for that purpose.
A can will last for years, you only need a few sprays to fix the OP-Z buttons, and I have no doubt that they will need redoing in the future, so it is handy stuff to have around.
Hope it helps, it worked well for me but be mindful to follow the instructions if you want to try it on yours, at your own risk.