Thank you very much for sharing this information!
Indeed I had exactly the same problem: suddenly some keys of my SU-42 keyboard were not working anymore (the ENTER, the 'p', the 'e', etc.). It was very strange because it did not receive any hit nor it didn't fall, so this happened suddenly without any special reason that I may be aware.
I first tried to uninstall and reinstall its drivers but it didn't work. Unfortunately it was quite clear that it was not a SW problem. I thought that maybe it was some kind of battery issue, but it didn't make any sense. So I was giving up when I had the chance to read this post, so I decided to follow the "oliverr871" and "derrickg02" indications.
The first step is to remove the cover and unfortunately in my case it was not so easy: I had to use an old plastic credit card to pull gently the back, little by little, starting in one of the USB's side, and moving around pulling all the cover. I had to do apply some strength until the internal tabs started to be removed with a peculiar "click" sound. Finally I could remove totally the back cover to access the electrical part of the backside of the keyboard.
In my case, I did not notice any problem with the black flat parallel cable and at least it seemed to me that it was perfectly connected to its mother connector. But as "derrickg02" recommended, I decided to remove it in order to clean it and then reconnect it to ensure that it was really correctly connected.
It is important to highlight (as "robertre" explains) that it is not needed to do any strength to unplug this flat cable. I was a little confuse because I thought that I had to pull up the connector in some way. Do not do that! It is much more simple. You only have to remove its orange transparent sticker and then you only need to slide gently the cable towards yourself. It has a white removable part that acts as clip and that will be removed too, so the black flat cable will be disconnected easily. I include some pictures to show how the cable is when it is removed from the connector.
So then I cleaned the connectors blowing on the connector to ensure that there was no dust inside. I put again the white removable clip together with the flat cable inside the connector. I pushed with a little of strength to ensure that now all the connectors where correctly connected again. To be sure, I connected the Lumia 2520 tablet and tested the keyboard before reassembling its back part. And yes, finally all the keys were working perfectly again! So I put again the orange sticker on top of the flat cable (at the end, I used a new sticker) to ensure that it was not moving away from the connector again, and then reassembled its back cover.
Now I'm enjoying again my great Nokia Lumia 2520 with its SU-42 keyboard working perfectly! I'm very happy because I have been using them for the last 5 years and they have been working as a charm until now. I know that such tablet is quite old for our nowadays standards, but the fact is that it has been working perfectly for my needs. Specially when I travel for meetings or on weekends and I don't want to carry my laptop. With the tablet battery and its keyboard one, I can use it few hours without any problem. I mainly use it to read emails, to take notes in OneNote or edit Office documents that I download from my OneDrive account. And of course, also to listen music, watch offline videos, or to browse Internet. Yes, I know what you are thinking: we are in 2020 and there are much better new W'10 tablets, but honestly, why do we have to renew our electronic devices so often if our actual devices work perfectly? In my humble opinion, it is a shame for those companies that use Marketing arguments to convince us to renew our devices each 3 or 4 years. Programmed obsolescence in software and hardware should be totally banned and forbidden. Nowadays it is very clear that such behavior is not sustainable for our environment. So here I am with my nice Lumia 2520 tablet hopefully for many years more!
Cheers!