@forno intermittent errors like this are hardest to diagnose. It could be a firmware issue. I would reset my phone first, if that does not help do a hard reset. You NEED to have all of your files,, pictures, password etc. backed up because a hard reset will delete those files. You can always check [https://www.hardreset.info/devices/samsung/samsung-galaxy-a20s/|on here] for the how-to.
If that does not fix your phone, it'll be the touch screen itself. Replacing it might be a solution. You will need to replace the complete display assembly since the touch screen is part of that. Use something like [https://www.youtube.com/watch?v=r4lbC8_O9hs|this video] as your guide
If that still won't work, it is time to do some checks on the board. Use the Flowchart for that
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While you work on your phone, take lots of pictures. Once you are done with it, take those pictures and create a guide for iFixit. That will help the next person that has the same problem as you do. It's easy and fun to create these guides. https://ifixit.com/Guide/new The iFixit community will appreciate this.
Repair is War on Entropy!